Support Python namespace packaging

[kylebarron]

The current state of the cookiecutter doesn't support Python's namespace packaging. The cookiecutter assumes that the Python package name coincides with the directory to Python files. For namespace packages this is not true. The Python package name may be something like [namespace].[module_name] while the physical directory layout would be [namespace]/[module_name]/.

I think the way to fix this would be to separate the existing python_package_name cookiecutter argument into two arguments: python_package_name and python_directory_name. The latter could be pre-filled with the value of the former, but it would make it possible for consumers to easily override directory paths to support namespace packaging.

(Note: I actually used the TS cookiecutter, but posting the issue here since this repo seems to have greater visibility).