extensions: make `hg nonexistent` not crash with PyOxidizer 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). Differential Revision: https://phab.mercurial-scm.org/D8750