setup: properly install build_hgextindex for py2exe builds Because the hgbuild class has a private copy of build.sub_commands, modifying build.sub_commands from this code effectively resulted in a no-op. Registering the sub-command on hgbuild actually results in the sub-command running when building Mercurial. Differential Revision: https://phab.mercurial-scm.org/D6093

setup: configure py2exe config via environment variables The Inno Setup and WiX installers ship a different set of packages with py2exe builds. And there are multiple WiX installer variants (e.g. TortoiseHG). Since there are multiple variants of py2exe configs and they can be defined by entities not in our repository, let's provide a mechanism for setup.py to supplement behavior via environment variables. This is slighly less hacky than a setup.cfg file IMO since the caller doesn't need to worry about mutating global state of the source directory. Differential Revision: https://phab.mercurial-scm.org/D6092