cmd/go: make 'go mod download' update go.sum after downloads are complete
'go mod download' calls WriteGoMod once via modload.ListModules when
it loads the build list. This saves sums for go.mod files needed by
MVS, but the write occurs before any zip files are downloaded.
With this change, 'go mod download' calls WriteGoMod again (and thus,
modfetch.WriteGoSum) after downloading and verifying module zip files,
so the sums of the zip files will be saved, too.
Fixes #41341
Change-Id: I7d56754aa255256ed45fd93cb154c2e6ea5f45a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/254357
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>