Support module and package names in the codemod context

[lpetre]

Summary

Relative imports are relative to the module's package, not the module's name.

Therefore we need to compute the package from the file.

Test Plan

~/EllipticalFrightenedOpposites$ tree -I __pycache__ x/
x/
├── __init__.py
└── y
    ├── __init__.py
    └── z.py

1 directory, 3 files

~/EllipticalFrightenedOpposites$ python3 -c 'import x.y.z as mod; print(mod.__package__, mod.__name__)'
x.y x.y.z

~/EllipticalFrightenedOpposites$ python3 -c 'import x.y as mod; print(mod.__package__, mod.__name__)'
x.y x.y

[zsol]

Nice.