extensions: load help from hgext.__index__ as a fallback this time Prior to 843418dc0b1b, `hgext.__index__` was consulted first if present, which caused the longer help from the extension modules to be ignored, even when available. But that change causes a bunch of test failures when the pyoxidized binary bundles *.pyc in the binary, saying the there's no help topic for `hg help $disabled_extension` and suggesting the use of `--keyword`, rather than showing a summary and indicating that it is disabled. Current failures were in test-check-help.t, test-extension.t, test-help.t, and test-qrecord.t. Ideally, we would read the various *.pyc files from memory and slurp in the docstring, but I know that they used to not be readable as resources, and I can't figure out how to make it work now. So maybe 3.9 and/or the current PyOxidizer doesn't support it yet. I got closer in py2exe with `importlib.resources.open_binary("hgext", "rebase.pyc")`, but `open_binary()` on *.pyc fails in pyoxidizer.[1] Either way, the *.pyc can't be passed to `ast.parse()` as `extensions._disabledcmdtable()` is doing, so I'm setting that aside for now. [1] https://github.com/indygreg/PyOxidizer/issues/649