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

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

Fixes #7638

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/87300043
»»»

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/87890043