imports: don't ignore GOPATH if GOROOT is a prefix of GOPATH

Fixes golang/go#16458

Change-Id: I1aaec5d115dd703dd702101f6bec37bf8b02a73d
Reviewed-on: https://go-review.googlesource.com/25192
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed