Globals (GHC boot packages) pruning

[qrilka]

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Tested manually, plan to run integration tests.
Failure to build global packages as targets results in 2 errors (using stack.yaml from #4510 with time overriden):

$ stack build directory --dry-run

Error: While constructing the build plan, the following exceptions were encountered:

Can't use GHC boot package directory when it has an overriden dependency, you need to add the following as explicit dependencies to the
project: 
 unix

Some different approaches to resolving this:

  * Consider trying 'stack solver', which uses the cabal-install solver to attempt to find some working build configuration. This can be
    convenient when dealing with many complicated constraint errors, but results may be unpredictable.


Plan construction failed.

or

$ stack build process --dry-run

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for process-1.6.5.0:
    directory must match >=1.1 && <1.4, but the stack configuration has no specified version  (latest matching version is 1.3.3.2)
    unix must match >=2.5 && <2.9, but the stack configuration has no specified version  (latest matching version is 2.7.2.2)
needed since process is a build target.

Some different approaches to resolving this:

  * Consider trying 'stack solver', which uses the cabal-install solver to attempt to find some working build configuration. This can be
    convenient when dealing with many complicated constraint errors, but results may be unpredictable.

  * Recommended action: try adding the following to your extra-deps in /home/qrilka/ws/h/stack4510/stack.yaml:

directory-1.3.3.2@sha256:79e15a62f90cb9f20f46b2b56fe67d00d71ed54cfad07ea337695f9ebb74baa4,2829
unix-2.7.2.2@sha256:4ef1d010d70a4a07a717e853d4a440c105dad38c6199316e320fdd4c48dacd34,3495

Plan construction failed.

Fixes #4510

Also I plan to update curator to take these details into account (probably as a separate PR)

[qrilka]

I have fixed integration test 345-override-bytestring and added this pruning to curator. It looks like build-constraints.yaml doesn't include parsec (while mtl is overriden) - PR on https://github.com/commercialhaskell/stackage will follow

[qrilka]

This package pruning implementation partially relies on information in global-hints.yaml but as e.g. Michael demonstrated in his blog post packages shipped with GHC could have no (or bad) corresponding version on Hackage so I'm changing it to to use correct package information from ghc-pkg

[qrilka]

I have changed the code quite a lot, also it looks like the refactoring I promised almost a month ago #4480 (comment) could be quite helpful in improving code around SkipCompilerDownload.
@snoyberg could I ask you for another look?

[snoyberg]

Just some minor comments.