python: add pkg_resources in the namespace __init__.py
python: add pkg_resources in the namespace __init__.py

For the __init__.py in the main package, this change follows
the advisory at
https://packaging.python.org/guides/packaging-namespace-packages/#pkg-resources-style-namespace-packages

to add the the line

    __import__('pkg_resources').declare_namespace(__name__)

to ONLY the main package's directory i.e. in the same directory
level as setup.py

Fixes #4

google/sqlcommenter,google/sqlcommenter,google/sqlcommenter,google/sqlcommenter,google/sqlcommenter,google/sqlcommenter,google/sqlcommenter