Before aliases, and because we chose a simple export format for them,
a package may now export the same object more than once if there are
multiple exported aliases referring to that object. The go/importer
made the assumption this couldn't happen. Adjust it.

Fixes #17726.

Change-Id: Ibb9fc669a8748200b45ad78934d7453e5a5aad82
Reviewed-on: https://go-review.googlesource.com/32538Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>