Because goimports also sorts stdlib-vs-external differently
(whereas gofmt just sorts string-wise), we need to put a blank
line between imports of different classes. This happened in
some cases, but not all.
Fixesgolang/go#7132
LGTM=kamil.kisiel
R=golang-codereviews, kamil.kisiel
CC=golang-codereviews
https://golang.org/cl/60870047
Fast path for the common case. Avoids scanning GOPATH usually.
LGTM=r, crawshaw
R=david.crawshaw, adg, r, crawshaw
CC=golang-codereviews
https://golang.org/cl/56820043