cmd/go: make malformed import path message more precise

cmd/go: make malformed import path message more precise

If you say 'go get -v' you get extra information when import
paths are not of the expected form.

If you say 'go get -v src/rsc.io/pdf' the message says that
src/rsc.io/pdf does not contain a hostname, which is incorrect.
The problem is that it does not begin with a hostname.

Fixes #7432.

LGTM=r
R=golang-codereviews, r
CC=bradfitz, golang-codereviews, iant
https://golang.org/cl/144650043