This turned out to be a black hole, however we believe we now have a
plan that does the right thing and shouldn't need to change again.
Error messages will only ever refer to a name in an unambiguous way,
falling back to <package>:<module>.<name> if no unambiguous shorter
variant can be found.  See HscTypes.mkPrintUnqualified for the
details.

Earlier hacks to work around this problem have been removed (TcSimplify).