Refactor checkpointable utilities to allow object graph customization

We want to add "virtual" objects like signatures before saving a SavedModel. Before this CL the saving utilities were a bit too literal for that to work.

Adds an ObjectGraphView, and gives one of those to the SavedModel _SaveableView. This will let the _SaveableView (or the thing that constructs it) add new edges that weren't in the saving program while making it clear that the same set of objects is going into the SavedModel and being saved in the checkpoint.

This CL isn't changing functionality, just shuffling it around a bit. I removed tf.contrib.eager.CheckpointableSaver as a symbol because its signature is changing and nobody is using it (that I can see); people can just use tf.train.Checkpoint. I've switched unit tests which were using it to using Checkpoint instead.

PiperOrigin-RevId: 232329203