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>

feature/plural/gen.go

diff --git a/internal/number/gen_plural.go b/feature/plural/gen.go
similarity index 100%
rename from internal/number/gen_plural.go
rename to feature/plural/gen.go

feature/plural/plural.go

diff --git a/internal/number/plural.go b/feature/plural/plural.go
similarity index 100%
rename from internal/number/plural.go
rename to feature/plural/plural.go

feature/plural/plural_test.go

diff --git a/internal/number/plural_test.go b/feature/plural/plural_test.go
similarity index 100%
rename from internal/number/plural_test.go
rename to feature/plural/plural_test.go