undo CL 169000043 / 05b838013df9

This was a mistake. The cmd/api tool
depends on an old version of go/types.

««« original CL description
cmd/api: use golang.org/x/... import paths

LGTM=bradfitz, rsc
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/169000043
»»»

TBR=rsc, bradfitz
R=bradfitz, rsc
CC=golang-codereviews
https://codereview.appspot.com/169320043