Fix a few minor issues spotted in code review
Add release notes about ghc-pkg change, and Cabal dep removal

That ghc-pkg doesn't support single-file style databases, and that
the ghc library does not depend on Cabal any more.

We don't need to document the ghc-pkg change in the ghc-pkg section
itself, since ghc-pkg init is already described there, and that is the
right thing. The old deprecated approach was not documented.