[Python APIView] create spec for detecting handwritten code and implement

Outline spec for correctly detect fully handwritten packages and implement.
Specifically:
- Check to see if any _patch.py files exist in public folder. Still need to account for workarounds like importing from a private folder (_generated).
Ex.
```py
# azure/foo/__init__.py
from ._generated.models import BarModel

# azure/foo/_generated/models.py
class BarModel:
...
```