Copy skcms public-API header into src directory.

The top-level `skcms.h` is still intended for external clients,
but it is just a simple wrapper around `src/skcms_public.h` now.

Change-Id: Ia58760fb2bce6db8675a7f1925a1d72324677338
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/774336
Reviewed-by: Herb Derby <herb@google.com>