extensions: avoid including `__index__` in the disabled extension list This generated module contains a dictionary of all bundled extension names and their help for builds that cannot enumerate extensions in the filesystem. The disabled list gets displayed in `hg help extensions`, and is also used by `setup.py` to populate `__index__.py` when building. I haven't seen it sneak into either py2exe or PyOxidizer builds, but it does show up when running tests locally after having created an installer. Differential Revision: https://phab.mercurial-scm.org/D9544