all: use golang.org/x/... import paths

LGTM=rsc, r
R=r, rsc
CC=golang-codereview, golang-codereviews
https://codereview.appspot.com/168050043
cmd/go: use golang.org/x/... import paths

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/168170043