go/internal/gccgoimporter: revise previous anonymous field fix.

Revise the fix for #31540 to take into account the possibility that we
may see an alias to a name that has already been installed into
package scope. This scenario is not possible to reproduce with the
current importer unit tests; changes to the harness to enable this
scenario will be added in a later CL.

Updates #31540.

Change-Id: Ie155d5e0b998604177a78471cba2413f57d40229
Reviewed-on: https://go-review.googlesource.com/c/go/+/173440


Reviewed-by: Ian Lance Taylor <iant@golang.org>