Summary:
Previously, we used $1_$2_PACKAGE_KEY to parametrize $1.  But the
documentation says that $1 should be the directory...  and we're now
putting the libraries in $1_$2_LIB_NAME.  So use /that/.  This is just
alpha-renaming, so as long as we're consistent, there's no material
difference.)

I also fixed a bug of a package ID calculation which I missed first
time around, which was tickled by this change.

BTW, this means DEP_KEYS and TRANSITIVE_DEP_KEYS are unused, so
remove them from ghc-cabal.

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

Test Plan: validate

Reviewers: austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1010