PDL: Improve the Python backend generator

- Fix specialization of packets when no constraints
  is provided (aliased packets or packets with no
  direct identification)
- Exclude payload from packet comparison
- Add post_init to set constraint values in child
  packet builder.

Test: atest --host pdl_python_generator_test
Change-Id: Ia1a1a90dac66cf43f8ea1b59854308d16aeaad24