Override python installation paths

The python module now has options to control where modules are installed:

• python.platlibdir: Directory for site-specific, platform-specific files.
• python.purelibdir: Directory for site-specific, non-platform-specific files.

Those options are used by python module methods python.install_sources() and python.get_install_dir(). By default Meson tries to detect the correct installation path, but make them relative to the installation prefix, which will often result in installed python modules to not be found by the interpreter unless prefix is /usr on Linux, or for example C:\Python39 on Windows. These new options can be absolute paths outside of prefix.