Support for Cabal 2.0 sub and foreign libraries

[snoyberg]

This should resolve both #3364 and #3361. There is a test case included
that should address both of them as well.

This is still relatively hacky. We will eventually be overhauling the
component system more dramatically to support Backpack with #2540 (CC
@ezyang).

This may still have some problems due to the upstream issue
haskell/cabal#4763, but at least in theory we're matching the behavior
of upstream. We can consider workarounds after that issue comes to a
conclusion.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

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.

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

There's a new integration test that adds a sub library and foreign library. Then I just kept smacking stack build and changing the code base until that built successfully.