distutils-common-base: move Python dir detection to separate class

distutils-common-base: move Python dir detection to separate class

Allows to inherit that in packages containing Python extensions but still tightly
control packaging of those extensions. Mainly for the cases where you want
those python modules/extensions/parts to be packaged separately from the main
package in order not to introduce python dependency where you don't need it.
Technically, you can do it with distutils-common-base, but it might require
more FILES_* work than with python-dir.

Introduce PYTHON_SITEPACKAGES_DIR along the way, site-packages directory is
referenced frequently enough within class files, but some recipes also pack this
directory as

${libdir}/python2.5/site-packages
${libdir}/python2.6/site-packages
${libdir}/python*/site-packages
/usr/lib/python*/site-packages
${libdir}/*/site-packages

all of which are not perfect.

Signed-off-by: Roman I Khimov <khimov@altell.ru>