Add explicit export lists to all exposed modules of base

There have been recent discussions of splitting base.

To keep base interface stable I think it's mandatory to actually explicitly specify it.

Otherwise if e.g. System.Posix.Internals is migrated to ghc-base or similar it may change in refactoring. (Any new helper function would be exposed!)

And even if base is not split any time soon, having the exposed interface explicit won't hurt.

EDIT: also GHC.Exts is a misnomer as it re-exports GHC.Types, GHC.Prim etc. Technically forcing base version bumps to be a superset of ghc-prim bumps.