Packaging: "Guard" calls to `setuptools.setup()` behind `if __name__ == '__main__'`

[dhermes]

This way our setup.py files could be imported (for example).

This is low-priority, but an easy change to make.

[tseaver]

I'm a pretty big -1 on the idea that setup.py should ever be importable.

[dhermes]

@tseaver I don't actually mind if it's import-able, but that was just an example. It's more just like good hygiene to have if __name__ == '__main__' in all scripts.

[duggelz]

+1 on __main__ hygiene across the board.