Switch from -this-package-key to -this-unit-id.

A small cosmetic change, but we have to do a bit of work to
actually support it:

    - Cabal submodule update, so that Cabal passes us
      -this-unit-id when we ask for it.  This includes
      a Cabal renaming to be consistent with Unit ID, which
      makes ghc-pkg a bit more scrutable.

    - Build system is updated to use -this-unit-id rather than
      -this-package-key, to avoid deprecation warnings.  Needs
      a version test so I resurrected the old test we had
      (sorry rwbarton!)

    - I've *undeprecated* -package-name, so that we are in the same
      state as GHC 7.10, since the "correct" flag will have only
      entered circulation in GHC 8.0.

    - I removed -package-key.  Since we didn't deprecate -package-id
      I think this should not cause any problems for users; they
      can just change their code to use -package-id.

    - The package database is indexed by UNIT IDs, not component IDs.
      I updated the naming here.

    - I dropped the signatures field from ExposedModule; nothing
      was using it, and instantiatedWith from the package database
      field.

    - ghc-pkg was updated to use unit ID nomenclature, I removed
      the -package-key flags but I decided not to add any new flags
      for now.

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

Test Plan: validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: 23Skidoo, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1780