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