packaging: extract py2exe functionality to own module 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.) Differential Revision: https://phab.mercurial-scm.org/D6091