This adds a comment in go/packages/external.go that specifies what the
driver protocol is.
Change-Id: Ie0c272a84cd34ba80f80f68b328463d8ddd07189
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184943
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>