undo CL 87300043 / 1dc800571456
undo CL 87300043 / 1dc800571456

This breaks "go get -d repo/path/...".

««« original CL description
cmd/go: do not miss an error if import path contains "cmd/something"

Fixes issue 7638

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/87300043
»»»

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/87890043

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