GHC 8.0 is switching the state sponsored way to specify
linker names from -this-package-key to -this-unit-id, so
it behooves us to use the right one.  But it didn't make
much sense to pass ComponentIds to a flag named UnitId,
so I went ahead and finished a (planned) refactoring
to distinguish ComponentIds from UnitIds.

At the moment, there is NO difference between a ComponentId
and a UnitId; they are identical.  But semantically, a
component ID records what sources/flags we chose (giving us enough
information to typecheck a package), whereas a unit ID records
the component ID as well as how holes were instantiated
(giving us enough information to build it.)  MOST code
in the Cabal library wants unit IDs, but there are a few
places (macros and configuration) where we really do
want a component ID.

Some other refactorings that got caught up in here:

    - Changed the type of componentCompatPackageKey to String, reflecting the
      fact that it's not truly a UnitId or ComponentId.

    - Changed the behavior of CURRENT_PACKAGE_KEY to unconditionally
      give the compatibility package key, which is actually what you
      want if you're using it for the template Haskell trick.  I also
      added a CURRENT_COMPONENT_ID macro for the actual component ID,
      which is something that the Cabal test-suite will find useful.

    - Added the correct feature test for GHC 8.0 ("Uses unit IDs").

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