cmd/go: add go mod download

go mod download provides a way to force downloading
of a particular module version into the download cache
and also to locate its cached files.

Forcing downloads is useful for warming caches, such as
in base docker images.

Finding the cached files allows caching proxies to use
go mod download as the way to obtain module files
on cache miss.

Fixes #26577.
Fixes #26610.

Change-Id: Ib8065bcce07c9f5105868ec1d87887ef4871f07e
Reviewed-on: https://go-review.googlesource.com/128355


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>