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>