ghc-package.eclass: hide all phase functions and reexport in haskell-cabal.eclass

Keeping in mind ghc-package_pkg_* functions is nonintuitive
when writing complex ebuilds. People always forget about it.

Reexported phases in haskell-cabal.eclass instead.
It breaks API, but in a minor way as only recaching
won't be done.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>