Refactor lookupExactOrOrig & friends
This refactors the panoply of renamer lookup functions relating to
lookupExactOrOrig
to more graciously handle Exact and Orig names.
In particular, we avoid the situation in which we would add Exact
/Orig
GRE
s to the tcg_used_gres
field, which could cause a panic in bestImport
like in #23240 (closed).