Fixes #3157. The wrapText helper is used to format all error
messages. Previously, it was only used to format IOException errors;
other exceptions would be formatted incorrectly.

Exporting the default rendering style allows us to consistently format
text without needing to write a Text instance.

Fixes #3214.

update -prof Check message

The flag enable-executable-profiling warns to change it to just enable-profiling. So we'll fix that here too so we don't send people down an old path.

Introduce UnresolvedSourcePackage alias

The alias is a shorthand for 'SourcePackage UnresolvedPkgLoc' which is
used all over the place.

Change Cabal's GHC support window to GHC 6.12 or later

Parameterize SourcePackage and ConfiguredPackage on package location

(Try to) add GHC 8.0.1 to the test-matrix

Currently GHC 8.0 has a slightly different error message if it can't
find a `.hi` file. It's not clear yet if and how we're going to change the
message before GHC 8.0 final.

This is a step towards breaking the dependency from the modular
solver on cabal-install.

Make the solver aware of pkg-config constraints

When solving, we now discard plans that would involve packages with a
pkgconfig-depends constraint which is not satisfiable with the current
set of installed packages (as listed by pkg-config --list-all).

This fixes https://github.com/haskell/cabal/issues/3016.

It is possible (in principle, although it should be basically impossible
in practice) that "pkg-config --modversion pkg1 pkg2... pkgN" fails to
execute for various reasons, in particular because N is too large, so
the command line becomes too long for the operating system limits.

If this happens, revert to the previous behavior of accepting any
install plan, regardless of any pkgconfig-depends constraints.

Refactor 'Explore.backjumpInfo' after code review

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

[ci skip]

Merge branch 'backjumping-space-leak' of https://github.com/grayjay/cabal into grayjay-backjumping-space-leak

Fix space leaks in dependency solver logging.

Previously, a setting of `--max-backjumps=-1` was reported in
the debug output as `0`. But `0` has a very different meaning,
and that is confusing.

(cherry picked from commit 9cd1387fcd582a6131746e6742597069d54f905a)

Improve goal reorder heuristics.

In GHC 8.0 -Wall implies -Wredundant-constraints, so we have to
address those warings in one way or another.

This change primarily does two things:

1. For `--reorder-goals`, we use a dedicated datatype `Degree`
   rather than an `Int` to compute the approximate branching
   degree. We map 0 and 1 to the same value. We then use a
   lazy ordering and a shortcutting minimum function to look
   for the "best" goal.

   The motivation here is that we do not want to spend
   unnecessary work. Following any goal that has 0 or 1 as degree
   cannot really be "wrong", so we should not look at any others
   and waste time.

   This will still not always make the use of `--reorder-goals`
   better than not using it, but it will reduce the overhead
   introduced by it.

2. We use partitioning rather than sorting for most of the other
   goal reordering heuristics that are active in all situations.
   I think this is slightly more straightforward and also slightly
   more efficient, whether `--reorder-goals` is used or not.

I have run some preliminary performance comparisons and they
seem to confirm that in both cases separately (with or without
`--reorder-goals`), these changes are a relative improvement
over the status quo. I will run additional tests before
merging this into master.

[ci skip]

Enable cabal-install integration tests in appveyor.yml