cmd/go: don't scan gccgo standard library packages for imports

In a gccgo installation the standard library sources are not available.

Change-Id: I929f3645e3ac95a1fa7047d6a3d243159a86ba66
Reviewed-on: https://go-review.googlesource.com/c/go/+/281493


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>