feature/plural: move plurals package

Preparing for making public.

Change-Id: I95b8f5ae1bce28ca0a144ae1693736e544ca19a2
Reviewed-on: https://go-review.googlesource.com/39131
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed