fix(cmake): findpython issues and 3.12 support for pybind11_find_import

[henryiii]

Fixing FindPython mode, which wasn't exporting the version number or Python_EXECUTABLE, which caused things like pybind11_find_import to break.

Also fixing pybind11_find_import to try importlib.metadata first, since pkg_resources is a third-party library and not always present, especially on Python 3.12.

Suggested changelog entry:

Fix FindPython mode exports & avoid ``pkg_resources`` if ``importlib.metadata`` available.