cmd/go: ignore import "C" files in module loader in non-cgo mode

Obviously, including files that import "C" when cgo is disabled is wrong.
The package load step correctly excludes them and finds no files at all,
which then causes a failure.

Fixes #26927.

Change-Id: I00e6d6450e783d467d20bde99e91240ecb0db837
Reviewed-on: https://go-review.googlesource.com/129062


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>