cmd/go: use workspace modules' go.sum files to check sums

By default, use workspace modules' go.sum files to check sums. Any
missing sums will still be written to go.work.sum

For #45713

Change-Id: I0f537602523dfec44d423c3c80c7ef396e1397b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/359478


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>