Start moving Checkpointable utilities toward core

Doesn't add to the public API yet, just shifts code around. Changes:
  - A tiny bit of renaming (to avoid having _Checkpoint and Checkpoint in the same file)
  - Removed the garbage collection decorator from a few tests due to the uuid4() garbage issue (apparently core tests get run on Python 2.7.9?)
  - Renamed "Object" to "CheckpointableObject" in the proto, since core protos have Java bindings and apparently Java had something else in mind for the keyword "Object" :)
but otherwise this is a pure move.

After this CL I'll propose adding tf.train.Checkpoint to the API (currently tf.contrib.eager.Checkpoint), move the utilities that are still in contrib/eager to their own contrib directory (there will be a few more misc. utilities for inspecting checkpoints and managing dependencies), get tf.train.Saver to read object-based checkpoints for compatibility, and work on Model.save_weights/load_weights.

PiperOrigin-RevId: 192646890