cmd/go: reject embedded files that can't be packed into modules

If the file won't be packed into a module,
don't put those files into embeds.
Otherwise people will be surprised when things work
locally but not when imported by another module.

Observed on CL 290709

Change-Id: Ia0ef7d0e0f5e42473c2b774e57c843e68a365bc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/290809


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>