[release-branch.go1] cmd/go: in go get, don't try to perform discovery on non-hosts

[release-branch.go1] cmd/go: in go get, don't try to perform discovery on non-hosts

««« backport 2a52a9484c10
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
https://golang.org/cl/5981057
»»»