rust-pyo3: change the name of the Python module to pyo3_rustext...
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.
