Summary:
Previously, the GHC API was "semi" wired-in: it was installed with a
version number, but that version number was hard-coded into the compiler
and it wasn't really possible to install other copies of the GHC API.
This patch makes the GHC API more similar to existing wired-in packages
such as ghc-prim, and will be helpful when we start extending the amount
of information passed to -package-name.

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

Test Plan: validate

Reviewers: simonmar, simonpj, hvr, austin

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D78