When the GOMOD value is the operating system's null device, there
isn't a main module and thus there cannot be module requirements.
Don't try to fill module cache, since running 'go mod download'
would cause a "no modules specified (see 'go help mod download')"
error to be printed, which is confusing.
Fixesgolang/go#35476
Change-Id: Ia1f5ee50797a4dc3e36c3b9773e19da781bd9d39
Reviewed-on: https://go-review.googlesource.com/c/tools/+/206886
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>