cmd/go: in go get, don't try to perform discovery on non-hosts
cmd/go: in go get, don't try to perform discovery on non-hosts

Before, "go get -v foo/bar" was assuming "foo" was a hostname
and trying to perform discovery on it. Now, require a dot in
the first path component (the hostname).

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5981057

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang