goimports to use the package name as a local qualifier in an import.
For example, if findImports("pkg", "X") returns ("foo/bar",
rename=true), then goimports adds the import line:
import pkg "foo/bar"
to satisfy uses of pkg.X in the file.
This change doesn't add any implementations of rename=true, though one
is sketched in a TODO.
LGTM=crawshaw
R=crawshaw, rsc
CC=bradfitz, golang-codereviews
https://golang.org/cl/76400050
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