cmd/go: avoid indexing modules in GOROOT

cmd/go: avoid indexing modules in GOROOT

Scanning GOROOT modules for changes appears to be causing Windows
builders to time out in x/tools tests. We may try a per-package index
instead, but for now just skip GOROOT modules (as we do for main
modules).

Fixes #53493.

Change-Id: Ic5bb90b4ce173a24fc6564e85fcde96e1f9b975f
Reviewed-on: https://go-review.googlesource.com/c/go/+/413634
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>