Build backend: Support stubs packages (#13563)
Stubs packages are different in that their name ends with `-stubs`,
their module is `<module name>-stubs` (with a dash, not the generally
legal underscore) and their modules contain a `__init__.pyi` instead of
an `__init__.py`
(https://typing.python.org/en/latest/spec/distributing.html#stub-only-packages).
We add support in the uv build backend by detecting the `-stubs` suffix.
Fixes #13546
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
