Support proxies written in Python that are embedded in a compiled module

Python modules added to a plugin via PYTHON_MODULES may create proxies using the decorators. These do not work as one might expect right now because the proxy is not hooked into the global registration mechanism since the import happens in the Python shell, not at plugin load time.

See https://discourse.paraview.org/t/how-embedding-python-source-as-modules/1978 for discussion.

Cc: @utkarsh.ayachit