Refactor RdrName.Provenance, to fix #7672
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.
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