extensions: make `hg help extensions` list disabled extensions with PyOxidizer

`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`!

Differential Revision: https://phab.mercurial-scm.org/D8758

setup.py

@@ -1695,6 +1695,9 b' if py2exeloaded:'
     if dllexcludes:
         py2exedllexcludes.extend(dllexcludes.split(' '))
 
+if os.environ.get('PYOXIDIZER'):
+    hgbuild.sub_commands.insert(0, ('build_hgextindex', None))
+
 if os.name == 'nt':
     # Windows binary file versions for exe/dll files must have the
     # form W.X.Y.Z, where W,X,Y,Z are numbers in the range 0..65535