cmd/go/internal/modload: skip go.mod directories on all systems, not just Plan 9

cmd/go/internal/modload: skip go.mod directories on all systems, not just Plan 9

I see no reason Plan 9 should be special cased.
A directory named go.mod is not useful on any system.
Followup to CL 129804.

Change-Id: I9cc91b5934b17650bfdb07370aa73aeae445968c
Reviewed-on: https://go-review.googlesource.com/c/149337
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>