This change adds a command mvpkg that will move a given package and
update all its imports. It uses similar logic to gorename to update
the imports.
Change-Id: Iebbd0b4c93c2302b0a71c3b99c68f6778106012a
Reviewed-on: https://go-review.googlesource.com/1973
Reviewed-by: Alan Donovan <adonovan@google.com>
