cmd: ignore the directory named go.mod

The existing implementation does not check in all cases whether go.mod is a regular file.

Fixes #30788

Change-Id: I6d140545c3cfada651612efd5bee2fbdcb747ca7
GitHub-Last-Rev: 4a9b251e378d9d7cc8768d395c360d3542fc9bc6
GitHub-Pull-Request: golang/go#30830
Reviewed-on: https://go-review.googlesource.com/c/go/+/167393


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>