This commit changes the way that the solver generates the summarized log that it
displays at normal verbosity.

Previously, the solver saved the full log from the start to the first backjump.
Then it filtered the log using the conflict set from the node where the first
backjump occurred, i.e., it removed all lines from the log that did not relate
to variables in the conflict set.  The solver also printed the final conflict
set at the end of the log.

This approach had several problems:

1. It was possible for the conflicts at the first backjump to be unrelated to
   the final conflict set (issue #941).  The conflicts in the summarized log
   could be irrelevant to the failure, for example, if they were caused by only
   a single version of a dependency, which the solver could skip, and the real
   problem was a different dependency that was missing from the index.  Even if
   the summarized log was relevant, showing two different explanations for the
   same failure could be confusing.

2. Filtering the full log was error prone and could remove the wrong lines.  It
   caused bugs mentioned in #2853 and #4154.

3. The conflict set at the first backjump contains the variables directly
   involved in the conflicts at that level and the variables that introduced
   them, but it doesn't contain the whole chain of variables starting with the
   user targets (issue #4792).  When the log is filtered with that conflict set,
   it can be unclear why the solver needed to choose the conflicting packages in
   the first place.

This commit creates the summarized log by rerunning the solver with a backjump
limit of zero and using the full log.  Using an unfiltered log avoids (2) and
(3).  However, it is also important to shorten the log by only showing choices
that are relevant to conflicts.  This commit uses different approaches for the
two types of solver failures.

No solution:

This commit makes the solver prefer variables from the first run's final
conflict set when choosing goals in the second run.  This means that the log to
the first backjump is more likely to be relevant to the final failure, because
it only mentions packages, flags, and stanzas from the final conflict set.

Backjump limit reached:

There is no final conflict set in this case, since the solver did not traverse
the whole tree.  This commit tries to create a final conflict set by rerunning
the solver with a subtree of the original search tree that contains the path to
the first backjump.  Then it uses the final conflict set from that run to
generate a log message, as in the case where the solver found that there was no
solution.

Here is an example of the differences between the new and old logs, using the
command from issue #4792 and GHC 8.2.1:

Before:

$ cabal install --dry-run --index-state=2018-01-04T21:05:55Z thorn
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: base-4.10.0.0/installed-4.1... (dependency of thorn)
next goal: profunctors (dependency of thorn)
rejecting: profunctors-5.2.1, profunctors-5.2, profunctors-5.1.2,
profunctors-5.1.1, profunctors-5.1, profunctors-5.0.1, profunctors-5.0.0.1,
profunctors-5 (conflict: thorn => profunctors<5)
trying: profunctors-4.4.1
next goal: transformers (dependency of profunctors)
rejecting: transformers-0.5.2.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: profunctors =>
transformers>=0.2 && <0.5)
rejecting: transformers-0.4.3.0, transformers-0.4.2.0 (conflict:
base==4.10.0.0/installed-4.1..., transformers => base>=2 && <4.9)
rejecting: transformers-0.4.1.0, transformers-0.3.0.0, transformers-0.2.2.1
(conflict: base==4.10.0.0/installed-4.1..., transformers +/-applicativeinbase
=> base>=1.0 && <4.8)
rejecting: transformers-0.2.1.0, transformers-0.2.0.0 (conflict:
base==4.10.0.0/installed-4.1..., transformers +/-applicativeinbase =>
base>=1.0 && <4.3)
rejecting: transformers-0.1.4.0, transformers-0.1.3.0, transformers-0.1.1.0,
transformers-0.1.0.1, transformers-0.0.1.0, transformers-0.0.0.0,
transformers-0.5.3.1, transformers-0.5.3.0, transformers-0.5.0.2 (conflict:
profunctors => transformers>=0.2 && <0.5)
rejecting: transformers-0.4.0.0 (conflict: base==4.10.0.0/installed-4.1...,
transformers +/-applicativeinbase => base>=1.0 && <4.8)
rejecting: transformers-0.2.2.0 (conflict: base==4.10.0.0/installed-4.1...,
transformers +/-applicativeinbase => base>=1.0 && <4.6)
rejecting: transformers-0.1.0.0 (conflict: profunctors => transformers>=0.2 &&
<0.5)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: transformers, contravariant, base,
thorn

After:

$ cabal install --dry-run --index-state=2018-01-04T21:05:55Z thorn
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: thorn-0.2 (user goal)
[__1] next goal: contravariant (dependency of thorn)
[__1] rejecting: contravariant-1.4, contravariant-1.3.3, contravariant-1.3.2,
contravariant-1.3.1.1, contravariant-1.3.1, contravariant-1.3,
contravariant-1.2.2.1, contravariant-1.2.2, contravariant-1.2.1,
contravariant-1.2.0.1, contravariant-1.2, contravariant-1.1, contravariant-1.0
(conflict: thorn => contravariant<1)
[__1] trying: contravariant-0.6.1.1
[__2] next goal: transformers (dependency of contravariant)
[__2] rejecting: transformers-0.5.2.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: contravariant =>
transformers>=0.2 && <0.5)
[__2] trying: transformers-0.4.3.0
[__3] next goal: base (dependency of thorn)
[__3] rejecting: base-4.10.0.0/installed-4.1... (conflict: transformers =>
base>=2 && <4.9)
[__3] rejecting: base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: transformers, contravariant, base,
thorn

Differences:

- The new summary has level numbers, like the full log.
- The conflicts are different.  The old log mentions thorn, base, profunctors,
  and transformers, and the new log mentions the four packages from the conflict
  set: thorn, contravariant, transformers, and base.
- The new log starts with the solver choosing a user goal, thorn.

The solver continues to display the conflicts at the first backjump when it
reaches the backjump limit, i.e, it shows profunctors instead of contravariant:

Before:

$ cabal install --dry-run --index-state=2018-01-04T21:05:55Z thorn --max-backjumps=10
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: base-4.10.0.0/installed-4.1... (dependency of thorn)
next goal: profunctors (dependency of thorn)
rejecting: profunctors-5.2.1, profunctors-5.2, profunctors-5.1.2,
profunctors-5.1.1, profunctors-5.1, profunctors-5.0.1, profunctors-5.0.0.1,
profunctors-5 (conflict: thorn => profunctors<5)
trying: profunctors-4.4.1
next goal: transformers (dependency of profunctors)
rejecting: transformers-0.5.2.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: profunctors =>
transformers>=0.2 && <0.5)
rejecting: transformers-0.4.3.0, transformers-0.4.2.0 (conflict:
base==4.10.0.0/installed-4.1..., transformers => base>=2 && <4.9)
rejecting: transformers-0.4.1.0, transformers-0.3.0.0, transformers-0.2.2.1
(conflict: base==4.10.0.0/installed-4.1..., transformers +/-applicativeinbase
=> base>=1.0 && <4.8)
rejecting: transformers-0.2.1.0, transformers-0.2.0.0 (conflict:
base==4.10.0.0/installed-4.1..., transformers +/-applicativeinbase =>
base>=1.0 && <4.3)
rejecting: transformers-0.1.4.0, transformers-0.1.3.0, transformers-0.1.1.0,
transformers-0.1.0.1, transformers-0.0.1.0, transformers-0.0.0.0,
transformers-0.5.3.1, transformers-0.5.3.0, transformers-0.5.0.2 (conflict:
profunctors => transformers>=0.2 && <0.5)
rejecting: transformers-0.4.0.0 (conflict: base==4.10.0.0/installed-4.1...,
transformers +/-applicativeinbase => base>=1.0 && <4.8)
rejecting: transformers-0.2.2.0 (conflict: base==4.10.0.0/installed-4.1...,
transformers +/-applicativeinbase => base>=1.0 && <4.6)
rejecting: transformers-0.1.0.0 (conflict: profunctors => transformers>=0.2 &&
<0.5)
Backjump limit reached (currently 10, change with --max-backjumps or try to
run with --reorder-goals).

After:

$ cabal install --dry-run --index-state=2018-01-04T21:05:55Z thorn --max-backjumps=10
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: thorn-0.2 (user goal)
[__1] next goal: profunctors (dependency of thorn)
[__1] rejecting: profunctors-5.2.1, profunctors-5.2, profunctors-5.1.2,
profunctors-5.1.1, profunctors-5.1, profunctors-5.0.1, profunctors-5.0.0.1,
profunctors-5 (conflict: thorn => profunctors<5)
[__1] trying: profunctors-4.4.1
[__2] next goal: transformers (dependency of profunctors)
[__2] rejecting: transformers-0.5.2.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: profunctors =>
transformers>=0.2 && <0.5)
[__2] trying: transformers-0.4.3.0
[__3] next goal: base (dependency of thorn)
[__3] rejecting: base-4.10.0.0/installed-4.1... (conflict: transformers =>
base>=2 && <4.9)
[__3] rejecting: base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
Backjump limit reached (currently 10, change with --max-backjumps or try to
run with --reorder-goals).

One downside of this change is that the solver may reach the backjump limit when
generating the summarized log, if the backjump limit is very low:

$ cabal install --dry-run --index-state=2018-01-04T21:05:55Z thorn --max-backjumps=1
Resolving dependencies...
cabal: Backjump limit reached (currently 1, change with --max-backjumps or try
to run with --reorder-goals).
Failed to generate a summarized dependency solver log due to low backjump
limit.

Another downside is the performance impact of rerunning the solver.  It looks
like there isn't a big change in run time when the solver finds a solution or
fails after an exhaustive search.  However, rerunning the solver to the first
backjump after it reaches the backjump limit can take a significant amount of
time.  The worst case I could find was acme-everything with GHC 7.10.3, where
that step took 13 seconds.  The difference was normally small, though.

I ran hackage-benchmark on packages from Hackage to try to find packages where
the run time changed by more than a few percent.  I stopped it after all
packages starting with "b" (That includes all uppercase packages).

compiler: GHC 8.2.1
index state: 2018-01-04T21:05:55Z
parameters: --min-run-time-percentage-difference-to-rerun=1 --pvalue=0.01 --trials=20 --print-skipped-packages

Out of 2219 packages, 1064 were skipped because the run times in the first trial
were within 1%, 1065 differed by more than 1% in the first trial but did not
show a significant difference in run time in 20 trials, and 90 did show a
significant difference in run time.  Here are the counts of packages for
different ranges of speedup, for those 90 packages:

speedup (master avg. run time / branch avg. run time)     package count
[0.93, 0.94)                                              1
[0.94, 0.95)                                              0
[0.95, 0.96)                                              0
[0.96, 0.97)                                              1
[0.97, 0.98)                                              7
[0.98, 0.99)                                              29
[0.99, 1.00)                                              47
[1.00, 1.01)                                              3
[1.01, 1.02)                                              2

The package with the biggest percentage change was bittorrent, which ran for
3.85 seconds on master and 4.12 seconds on this branch.  It reached the backjump
limit.