Split dependency tracking out from CheckpointableBase

Some unit test fiddling, but otherwise just moving code around.

My goal is to be able to use checkpointable data structures (or something like them) in Checkpointable's __setattr__ override. Checkpointable data structures depend on Layer, so Checkpointable and CheckpointableBase need to be in seprate files (so we can have the dependency chain CheckpointableBase->Layer->CheckpointableDataStructure->Checkpointable).

This will also require changes to python/keras/engine/__init__.py (which currently requires Network and Layer be imported together), but I'll do that in a separate change.

PiperOrigin-RevId: 201712549