cmd/go: avoid generating "malformed module path" errors for standard-library paths
If the path looks like it belongs in GOROOT/src and isn't there, we
should mention that in the error message — instead of the fact
that the path is not a valid module path, which the user likely
already knows.
Fixes #34769
Fixes #35734
Change-Id: I3589336d102e420a5ad3bf246816e29f3cbe6d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/210339
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>