language/display: moved display package

The display package is now limited to contain only names for values
related to the language package. Moving it as a subpackage of
language makes both its purpose and discoverability clearer.

Sorry for the inconveniences.

Change-Id: I5321e7b81f0837f25f2523f6eb4811f82d0455b8
Reviewed-on: https://go-review.googlesource.com/17490
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
10 files changed