cmd/go: new tag selection logic

cmd/go: new tag selection logic

The new logic is "use go1 if it's there, otherwise no tag."
Nothing needs to say "I require go1.0.1", and I want to
preserve some flexibility in defining what tags mean.

Right now (before go1.0.1) there is only one possible tag,
"go1", and I'd like to keep it that way.

R=golang-dev, bradfitz, r, adg
CC=golang-dev
https://golang.org/cl/6112060