py2exe builds are shared between Inno Setup and WIX. We'll
want the logic for performing py2exe builds to be reusable
across the code for both installers.
This commit extracts the py2exe-specific functionality into
its own module.
There's definitely room to customize things further. This will
be done in future commits, as necessary. (I'm not even sure what
customizations WIX will require yet. Presumably a lot.)