cmd/mod/edit: add -tool and -droptool support

For golang/go#48429

Change-Id: I1a7bd8ffddbc65e3b687dc1d40f3853702e1b5dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/521958


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>