Add -XPackageImports, new syntax for package-qualified imports

Add -XPackageImports, new syntax for package-qualified imports

Now you can say

  import "network" Network.Socket

and get Network.Socket from package "network", even if there are
multiple Network.Socket modules in scope from different packages
and/or the current package.

This is not really intended for general use, it's mainly so that we
can build backwards-compatible versions of packages, where we need to
be able to do

module GHC.Base (module New.GHC.Base) where
import "base" GHC.Base as New.GHC.Base