go/importer: always handle forward-declared imports in export data

go/importer: always handle forward-declared imports in export data

The textual export data generated by gc sometimes contains forward
references of packages. In rare cases such forward-referenced packages
were not created when needed because no package name was present.

Create unnamed packages in this case and set the name later when it
becomes known.

Fixes #13566.

Change-Id: I193e0ec712e874030b194ab8ecb3fca140f7997a
Reviewed-on: https://go-review.googlesource.com/18301
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>