go/types, types2: better error messages for invalid qualified identifiers
This change borrows code from CL 631356 by Emmanuel Odeke (thanks!).
Fixes #70549.
Change-Id: Id6f794ea2a95b4297999456f22c6e02890fce13b
Reviewed-on: https://go-review.googlesource.com/c/go/+/662775
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Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Mark Freeman <mark@golang.org>