[dev.cmdgo] cmd/go: maintain a go.work.sum file

This change causes the go command to maintain a separate go.work.sum
file when in workspace mode rather than using the go.sum files
from the individual modules. This isn't quite what the proposal
spec specifies, which is that the sums that don't exist in any
of the workspace modules are added to go.work.sum rather than
the necessary sums. That will be done in a future change.

Change-Id: I528b9b153a93a4cd67c5af471ad6d5bd3628578b
Reviewed-on: https://go-review.googlesource.com/c/go/+/334939


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