cmd/go: add support for go get -tool

cmd/go: add support for go get -tool

Running `go get -tool example.com/m1` will add a tool line to your mod
file and add any missing dependencies.

Running `go get -tool example.com/m1@none` will drop the tool line from
your mod file.

For golang/go#48429

Change-Id: I07b4776f1f55eff588d08cb6649d94cc42a729d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/563175
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>