cmd/bundle: add flag to rewrite golang.org/ to golang_org/ in imports

I've been doing this by hand since Go 1.7rc2.

Updates golang/go#16333

Change-Id: Ib12c013b14210123d48d6ad78922caf1286c20cc
Reviewed-on: https://go-review.googlesource.com/29086
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed