Create a skeleton tf.contrib.checkpoint.

My plan for this is to incubate tools for working with object-based checkpoints:
  - Tools for managing dependency graphs, e.g. checkpointable lists/dictionaries
  - Inspecting/visualizing checkpoints
  - Listing variables and gathering initializers from a Checkpointable object
    and its dependencies
  - Verifying all variables are accessible as dependencies, which should make
    converting existing graph building Saver uses easier/safer.

This CL includes none of those things, it just moves the split_dependency tool
here instead of contrib/eager.

PiperOrigin-RevId: 193531292