cmd/go: download fewer dependencies in 'go mod download'
In modules that specify 'go 1.17' or higher, the go.mod file
explicitly requires modules for all packages transitively imported by
the main module. Users tend to use 'go mod download' to prepare for
testing the main module itself, so we should only download those
relevant modules.
In 'go 1.16' and earlier modules, we continue to download all modules
in the module graph (because we cannot in general tell which ones are
relevant without loading the full package import graph).
'go mod download all' continues to download every module in
'go list all', as it did before.
Fixes #44435
Change-Id: I3f286c0e2549d6688b3832ff116e6cd77a19401c
Reviewed-on: https://go-review.googlesource.com/c/go/+/357310
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org>