It turns out that we don't really need to be able to
extract a ComponentId from UnitId, except in one case.
So compress UnitId into a single FastString.

The one case where we do need the ComponentId is when
we are compiling an instantiated version of a package;
we need the ComponentId to look up the indefinite
version of this package from the database.  So now we
just pass it in as an argument -this-component-id.

Also: ghc-pkg now no longer will unregister a package if
you register one with the same package name, if the
instantiations don't match.

Cabal submodule update which tracks the same data type
change.

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