Introduce `Distribution.Compat.Prelude.Internal` + WARNING

This unexposes `Distribution.Compat.Prelude` again, and adds
an exposed `.Internal` module which reexports the `.Prelude` module with
an attached module-level `WARNING` pragma.

This way users are discouraged to use this in `Setup.hs` files as they'd
see compile warning like:

    Foo.hs:19:1-55: warning: [-Wdeprecations]
      Module ‘Distribution.Compat.Prelude.Internal’:
        This modules' API is not stable. Use at your own risk, or better yet, use @base-compat@!

In `cabal-install` we import the `.Internal` module exactly once, and
use -Wno-deprecations to suppress that the warning of that single
import.