In Explore.hs, the solver calculates a conflict set corresponding to the option
to not choose a value for each goal, called 'initial'.  That conflict set should
be added to the ConflictMap when the current goal causes a conflict.  However,
there was a bug, and the solver added 'initial' to the ConflictMap at every
level.

The bug meant that the solver preferred all goals that it had already chosen.
If a new goal started to cause conflicts, it would take a while for the solver
to start preferring the new goal over the ones that it had previously seen.
See #4595 for an example where this problem caused longer backjumps and slowed
down the solver.

Testing

I ran "cabal install --dry-run" for all packages on Hackage with master and the branch with a 90 second timeout, GHC 8.0.1, and index state 2017-07-08T04:20:18Z. Then I filtered out all packages where both runs had the same result (success or failure) and the times were within 10%. I repeated that process three times to eliminate packages that had different run times due to noise. Then I ran "cabal install --dry-run --max-backjumps=-1" on the remaining packages and averaged three runs.

Since this branch changes a goal-ordering heuristic, I expected a lot of changes in run time in either direction. Out of 105 changes, 83 were faster, 20 were slower, and 2 involved a timeout on both branches. (The two that timed out had different run times before I removed the backjump limit.)

package                          master result   master time (seconds)   branch result   branch time (seconds)   speedup
AesonBson                        solution         3.06                   solution         2.59                    1.18
DisTract                         no solution      1.48                   no solution      1.34                    1.1
GPipe-Examples                   timeout         90.01                   no solution      2.11                    -
GuiTV                            solution         2.54                   solution        16.05                    0.16
Phsu                             solution         3.49                   solution         1.83                    1.9
Ranka                            no solution      2.05                   no solution      1.65                    1.25
Rlang-QQ                         no solution      2.97                   no solution      1.89                    1.57
SourceGraph                      timeout         90.04                   no solution     11.53                    -
Validation                       no solution      2.1                    no solution      6.59                    0.32
WebCont                          no solution      2.04                   no solution      1.6                     1.28
acme-everything                  timeout         90.02                   timeout         90                       -
aeson-t                          solution         2.04                   solution         1.81                    1.13
alsa-gui                         no solution      1.62                   no solution      1.86                    0.87
aviation-cessna172-diagrams      no solution      2.56                   no solution      2.29                    1.12
aws-dynamodb-conduit             no solution      2.22                   no solution      1.83                    1.22
azure-servicebus                 no solution      5.18                   no solution      2.87                    1.8
bamboo-theme-mini-html5          no solution      2.51                   no solution      2                       1.26
bitcoin-payment-channel          solution         3.42                   solution         2.73                    1.25
bittorrent                       no solution      2.75                   no solution      2.4                     1.15
blaze-builder-enumerator         no solution      1.62                   no solution      1.88                    0.86
blunt                            solution         2.99                   solution         2.5                     1.19
cash                             no solution      1.33                   no solution      1.53                    0.87
cheapskate-terminal              no solution      2.04                   no solution      1.62                    1.26
clckwrks-plugin-bugs             no solution      3.85                   no solution      5.78                    0.67
cmdtheline                       no solution      1.96                   no solution      1.63                    1.2
colchis                          no solution      1.99                   no solution      2.23                    0.89
collada-types                    no solution      1.86                   no solution      1.66                    1.12
convertible-text                 solution         1.61                   solution         1.83                    0.88
cqrs-example                     no solution      2.64                   no solution      2.32                    1.14
dixi                             solution         4.44                   solution         4.03                    1.1
ethereum-client-haskell          no solution      1.84                   no solution      2.45                    0.75
flowdock                         no solution      2.46                   no solution      3.19                    0.77
geniserver                       no solution      5.34                   no solution      4.84                    1.1
ghc-imported-from                solution         4.59                   solution         2.9                     1.58
gps2htmlReport                   solution         3.08                   solution         5.58                    0.55
guarded-rewriting                solution         1.54                   solution         1.34                    1.14
hack2-handler-happstack-server   no solution      1.76                   no solution      2                       0.88
halma-telegram-bot               solution         4.27                   solution         3.37                    1.27
happs-tutorial                   no solution      2.08                   no solution      1.86                    1.12
happstack                        no solution      3.08                   no solution      5.86                    0.53
happstack-facebook               timeout         90.01                   timeout         90.03                    -
haskelldb-hsql-mysql             no solution      1.73                   no solution      1.5                     1.16
hdbi                             no solution      1.92                   no solution      1.66                    1.15
hdbi-postgresql                  no solution      3.05                   no solution      1.95                    1.56
hdbi-sqlite                      no solution      1.91                   no solution      1.69                    1.13
hexpat-iteratee                  no solution      2.61                   no solution      1.84                    1.42
hist-pl                          no solution      2.69                   no solution      2.36                    1.14
hscd                             no solution      2.53                   no solution      1.59                    1.59
http-client-lens                 no solution      3.63                   no solution      1.96                    1.85
hubris                           no solution      3.62                   no solution      1.63                    2.22
infinity                         no solution      1.34                   no solution      1.5                     0.89
iteratee-parsec                  no solution      2.01                   no solution      1.74                    1.16
json-togo                        no solution      1.86                   no solution      1.65                    1.13
lat                              no solution      2.87                   no solution      1.86                    1.55
liquidhaskell                    solution         3.34                   solution         2.25                    1.48
manatee-core                     no solution      1.79                   no solution      1.6                     1.12
manatee-curl                     no solution      8.44                   no solution      2.73                    3.09
manatee-editor                   no solution      5.05                   no solution      2.81                    1.8
manatee-filemanager              no solution     29.09                   no solution      2.96                    9.82
manatee-imageviewer              no solution      1.78                   no solution      1.57                    1.13
manatee-mplayer                  no solution      8.98                   no solution      4.05                    2.22
manatee-terminal                 no solution      3.19                   no solution      2.41                    1.32
minimung                         no solution      1.86                   no solution      1.57                    1.19
monoids                          no solution      3.02                   no solution      2.68                    1.13
mprover                          no solution      1.89                   no solution      1.54                    1.23
ms                               no solution      8.04                   no solution      4.35                    1.85
music-sibelius                   solution         3.1                    solution         2.5                     1.24
nerf                             solution        22.54                   solution         3.09                    7.29
nomyx-api                        solution         5.46                   solution         4.39                    1.24
nomyx-library                    solution         2.08                   solution         1.86                    1.11
nomyx-server                     no solution      4.83                   no solution      3.92                    1.23
opaleye-classy                   no solution      2.07                   no solution      3.58                    0.58
openflow                         no solution      1.95                   no solution      1.66                    1.18
ot                               no solution      3.37                   no solution      1.95                    1.73
paypal-api                       no solution      1.89                   no solution      1.64                    1.15
pdf-slave-server                 no solution      3.21                   no solution      2.15                    1.49
phooey                           solution         2.55                   solution        16.24                    0.16
pipes-cereal-plus                solution         1.85                   solution         1.65                    1.12
pocket-dns                       no solution      2.71                   no solution      2.08                    1.3
pontarius-mediaserver            no solution      2.29                   no solution      2.7                     0.85
precis                           no solution      2.53                   no solution      3.23                    0.78
prove-everywhere-server          no solution      2.19                   no solution      1.92                    1.14
quickbooks                       no solution      5.86                   no solution      5.17                    1.13
rbpcp-api                        solution         2.35                   solution         2.13                    1.11
react-haskell                    timeout         90.02                   no solution     71.21                    -
regex-xmlschema                  no solution      1.34                   no solution      1.51                    0.89
remote-json-server               solution         2.1                    solution         1.8                     1.16
scholdoc-citeproc                no solution      3                      no solution      1.92                    1.57
scion-browser                    no solution      5.21                   no solution      4.65                    1.12
semdoc                           no solution      4.21                   no solution      3.04                    1.39
servant-auth-token-rocksdb       no solution      4.68                   no solution      2.32                    2.02
snaplet-auth-acid                no solution      2.25                   no solution      1.99                    1.13
snaplet-stripe                   no solution      3.46                   no solution      2.97                    1.16
sssp                             no solution      3.3                    no solution      2.79                    1.18
target                           solution         2.8                    solution         2.11                    1.32
tls-extra                        no solution      2.74                   no solution      2.36                    1.16
twentefp-rosetree                no solution      1.69                   no solution      1.97                    0.86
twitter-enumerator               no solution     28.39                   no solution      2.16                   13.15
wai-middleware-cache             no solution      1.95                   no solution      1.56                    1.25
wai-middleware-catch             no solution      1.9                    no solution      1.54                    1.24
wai-middleware-route             solution        11.9                    solution         1.62                    7.34
xml2json                         no solution      2.27                   no solution      1.68                    1.35
yesod-auth-account-fork          solution         3.33                   solution         2.87                    1.16
yesod-comments                   no solution     25.6                    no solution     14.44                    1.77
yesod-pure                       solution         7.99                   solution         4.45                    1.79

Here were the root causes:

- Some tests involving Custom setpu showed MORE output
  (UseLocalPackageForSetup) when run on GHC 8.2.  This is because GHC
  8.2 ships a recent enough version of Cabal to know how to emit
  markers, which means we have started picking up the output.  I hacked
  up these tests to not accept this output, but a more correct thing to
  do is figure out how to NOT request marking of a Setup script which is
  not the inplace install.  This was a little tricky so I bailed.

- GHC 8.2 no longer emits "It is a member of the hidden package".
  This broke CustomWithoutCabalDefaultMain.  Not sure if this
  is a GHC regression but it's pretty harmless.

- While I was at it, I fixed an inexhaustive pattern match in
  cabal-testsuite (though perhaps poorly; I couldn't figure out
  what the new constructor does.)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

This commit adds "cabal-version: >= 1.20" to enable per-component build and
work with all Cabal versions that support new-build.  It also adds the
"default-language" field, in order to avoid warnings.