rust-pyo3: change the name of the Python module to pyo3_rustext Two reasons for doing so - `pyo3-rustext` is not a valid Python identifier, hence the module could only be imported with `__import__("mercurial.pyo3-rustext")` and then retrieved by getattr. Normal usage is throught `mercurial.policy`. Hovever manual testing for development and perhaps unit tests can benefit a more natural way of doing. - The module's `__name__` attribute was already `pyo3_rustext`. This can lead to some discrepancies, in particular when we introduce submodules.