go/importer: use correct path when checking if package was already imported

go/importer: use correct path when checking if package was already imported

The importer uses a global (shared) package map across multiple imports
to determine if a package was imported before. That package map is usually
indexed by package (import) path ('id' in this code). However, the binary
importer was using the incoming (possibly unclean) path.

Fixes #15517.

Change-Id: I0c32a708dfccf345e0353fbda20ad882121e437c
Reviewed-on: https://go-review.googlesource.com/23012
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>