Solver: Enforce dependencies on libraries (fixes #779).

[grayjay]

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

---

The first two commits only modify the tests:

Solver DSL: Support benchmarks, internal libraries, and foreign libraries.

This change allows the solver quickcheck tests to test all types of components.
Previously, the tests generated the components in the solver DSL, but the code
that converted the packages to GenericPackageDescriptions removed some of the
components, i.e., it filtered out benchmarks and foreign libraries and merged
internal libraries with the main library.

Solver DSL: Support packages without libraries.

The solver DSL previously always added a default library component.

Solver: Enforce dependencies on libraries (fixes #779).

This commit generalizes the fix for issue #4781
(e86f838) by tracking dependencies on
components instead of dependencies on executables. That means that the solver
always checks whether a package contains a library before using it to satisfy a
build-depends dependency. If a version of a package doesn't contain a library,
the solver can try other versions. Associating each dependency with a component
also moves towards the design for component-based dependency solving described
in issue #4087.

/cc @kosmikus

[23Skidoo]

Thanks for doing this! @fgaz, I think this is relevant to your GSoC project.

[23Skidoo]

Looks good from what I can tell, please feel free to merge unless @kosmikus objects.

[23Skidoo]

Merged, thanks! Can you please also add a changelog note?

[kosmikus]

Superficially looks good to me as well, but I haven't looked at the changes in very much detail. Thanks.

[grayjay]

Thanks! I updated the changelog in #5316.

[hvr]

@grayjay is this expected to fix issues like with the following situation where build-depends refers to a package whose library component is unavailable, e.g.:

cabal-version:2.2
name: zzz
version: 0

library
  build-depends: http-client == 0.5.12.1 || == 0.5.12

$ cabal new-build -w ghc-7.8.4 --index-state='2018-05-09T06:20:23Z'
Error:
    Dependency on unbuildable library from http-client
    In the stanza 'library'
    In the inplace package 'zzz-0'

where I'd expect the cabal solver to backtrack, and find the solution with http-client-0.5.12

[grayjay]

No, this change only checks whether the library stanza exists. Checking that the library is buildable will probably require implementation of the "Component on/off" variable from #4087 (comment).

[hvr]

@23Skidoo I had to cherry-pick this one to 2.2 as well as #5337 needed this to cherry-pick cleanly to 2.2

See fe2110d & 4bdf0aa