cmake,debian: install pure python module to deb_system path

cmake,debian: install pure python module to deb_system path

in ubuntu 22.04 and debian unstable, the layout (scheme) for system
python module is named "deb_system", the default one is 'posix_local'.
and 'posix_local' installs python modules into paths like
usr/local/lib/python3.10/dist-packages/. hence dh_install fails
when it tries to find the files to be packaged under directory of
usr/lib/python3*/site-packages/.

in this change, the "deb_system" scheme is used if it is available,
and fall back to "posix_prefix" to be backward compatible with older
debian (derivative) distros.

also, update the source directories of pure python's installation
from `site-packages` to `*-packages`, to be compatible with ubuntu focal
and ubuntu jammy. as we are now using the specified scheme instead of
the default one.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>