cmd/go/internal/modload: avoid loading the full module graph to determine...

cmd/go/internal/modload: avoid loading the full module graph to determine which checksums to add to go.sum

For #36460

Change-Id: I606314054bd9064f7c4053f56049fabbaec54143
Reviewed-on: https://go-review.googlesource.com/c/go/+/309189


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