Move PackageFixedDeps from Cabal to cabal-install

The fundamental difference between Cabal and cabal-install is that the former
deals with installed libraries, and -- in principle -- knows about _library_
dependencies only, whereas the latters deals with setup, executable, test-suite
and benchmark dependencies in addition to library dependencies. Currently we
classify all of these simply as 'dependencies' but that will change shortly.

In this commit we take a first step towards this by moving the PackageFixedDeps
class, which deals with dependencies of packages rather than installed
libraries, from Cabal to cabal-install.

The commit is pretty simple; we just move the type class and update import
statements where necessary.