cmd/go: in module mode, populate PackagePublic.Root with the module root

'go test' uses the Root field to determine the set of files that
invalidate test results, and there is no other sensible meaning of
“root” for code within a module.

Fixes #29111

Change-Id: Icf1be90a26d22665613e42cb968087b63c36e74c
Reviewed-on: https://go-review.googlesource.com/c/go/+/154100


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