Add a way to generate compat.v* modules in create_python_api.py. We

need to generate V2 and V1 in one step since V2's "compat" module (e.g.
"tf.compat.as_bytes") overlaps with "compat.v1". That is, some of the __init__.py files
are *shared*. So, we need to run create_python_api.py just once to generate all.
A few additional changes:
- Split out a few functions to prevent them from growing too large.
- Added output_init_files_test.py to test consistency of api_init_files.bzl and
  api_init_files_v1.bzl. Before, this check was done in create_python_api.py
  but now that we have different files for V1 and V2 this is getting
  cumbersome, so I split it in a separete test.

PiperOrigin-RevId: 207629449