[release-branch.go1.16] cmd/go: in 'go mod download' without args, don't save module zip sums

'go mod download' without arguments is frequently used to populate the
module cache. It tends to fetch a lot of extra files (for modules in
the build list that aren't needed to build packages in the main
module). It's annoying when sums are written for these extra files.

'go mod download mod@version' will still write sums for specific
modules in the build list. 'go mod download all' still has the
previous behavior.

For now, all invocations of 'go mod download' still update go.mod and
go.sum with changes needed to load the build list (1.15 behavior).

Fixes #46214

Change-Id: I9e17d18a7466ac7271a0e1a2b663f6b3cb168c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/318629


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 4fb10b21)
Reviewed-on: https://go-review.googlesource.com/c/go/+/321892
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>