When running hg nonexistent, we try to look for extensions that
provide that command. We do that by looking for files in the
hgext.__file__ directory. However, PyOxidizer doesn't provide a
__file__, so we crash when running with PyOxidizer.
We should be able to look for the command in built-in extensions, but
we seem to already have code for skipping the scan when running in a
frozen binary, so I just modified that code instead.
By the way, it also seems like we should be able to search for
extensions in the hgext3rd module, but we don't do that yet either
(before or after this patch).