cmd/go: preserve sums for indirect deps fetched by 'go mod download'

Previously, commands that wrote go.sum (except 'go mod tidy') would
retain sums for zip files of directly required modules. Sums of
indirect dependencies wouldn't be retained unless they were used to
load packages.

With this change, sums for indirect dependencies will be retained if
they're available. This allows users to add missing sums with
'go mod download example.com/mod', which previously only worked for
directly required modules.

Note that 'go mod download' without arguments now adds sums for every
module in the build list. That matches 1.15 behavior.

For #41103

Change-Id: I4cce2bf1c73578dae836bdb5adb32da071554f1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/282692


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