Update Privateer's build system
This merge request updates Privateer's build system to use the pip and scikit-build-core style from NucleoFind. Notes:
- This new build system creates both a Python module (privateer_core.cpython...) and an executable (privateer)
- The executable is copied into the Python sitepackages so it is in the users path, but should really go wherever CCP4 executables go (defined by CMAKE_INSTALL_PREFIX?)