Remove hourglass imports for python/keras/engine/...

Previously there was python/keras/engine/__init__.py, which required every import of Layer (for example) to also import Network (since Python will not import a file in a directory without running its __init__.py file).

I'm working on adding automatic attribute tracking to Checkpointable (and Network) and so need to insert things between Layer and Network.

PiperOrigin-RevId: 201743850