unicode/cldr: moved cldr to the unicode directory

The cldr package is quite low-level and probably would only for
internal use if it hadn't predated the internal directory convention.

Added doc.go in unicode directory to clarify nature of packages in
this subdirectory.

After this, the package that remains to be moved to unicode is bidi.

Change-Id: Ibff3285ca8cc00980e48fd30754c3f7abc87b2a0
Reviewed-on: https://go-review.googlesource.com/17491
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
24 files changed