godoc/dl: serve "go get" meta for golang.org/dl/gotip
godoc/dl: serve "go get" meta for golang.org/dl/gotip

Change-Id: I05db10e9b3f4983d23af4dc5fd4cce9b3979e9c5
Reviewed-on: https://go-review.googlesource.com/c/153097
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools