Trac #7672 has a data type T in module A that is in scope
*both* locally-bound *and* imported (with a qualified) name.
The Provenance of a GlobalRdrElt simply couldn't express that
before. Now you can.

In doing so, I flattened out Provenance into GlobalRdrElt,
so quite a lot of modules are touched in a not-very-interesting
way.