Move ConstraintSource and ResolverPackage to Distribution.Solver.*

[BardurArantsson]

As a follow-up to #3381 these commits move ConstraintSource and ResolverPackage too.

Incidentally, ResolverPackage is a bit anemic as a module, but I've decided to stick with the general pattern of Distribution.Solver.Types.* for now. I think it'll be easier to see if there's anything that should perhaps be re-combined later on.

[BardurArantsson]

/cc @kosmikus @23Skidoo @grayjay @ezyang @dcoutts @grayjay @hvr @edsko

... just in case anyone's working on solver-relevant things and needs to know.

[BardurArantsson]

This leaves the following data types:

• DependencyResolver
• InstalledPreference
• LabeledPackageConstraint
• PackageConstraint
• PackagePreferences

which are required by the (modular) solver. Once the rest are moved, I'll start to investigate what's needed for the tests and perhaps see if TopDown can be moved sensibly (my hunch is still 'no').

The remaining imports are:

$ grep -r Distribution.Client Distribution/Solver
Distribution/Solver/Modular.hs:import Distribution.Client.Dependency.Types
Distribution/Solver/Modular/Solver.hs:import Distribution.Client.Dependency.Types
Distribution/Solver/Modular/Preference.hs:import Distribution.Client.Dependency.Types

So all Solver -> Distribution.Client.Types dependencies have at least been eliminated.

(Just FYI: I probably won't start up on this again for at least a few days.)

[BardurArantsson]

Ah, yes, CI failed because Hackage is currently down.

[BardurArantsson]

Going to redo the close->open dance since Hackage should be up now...

[kosmikus]

LGTM.

[kosmikus]

@23Skidoo Feel free to merge if you also don't see any problems.

[23Skidoo]

Merged, thanks!