setup.py has a mechanism for generating a list of bundled extensions
and their documenation and putting it in a hgext/__index__.py
file. That file is then read from extensions.py, which makes `hg
help extensions` work in py2exe binaries. This patch makes it so
setup.py produces the hgext/__index__.py file also when called by
PyOxidizer.
Thanks to Matt Harbison for pointing out the existence of
__index__.py!