python: split Makefile into smaller files

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python: remove explicit compile rule; works without it

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python: remove some explicit rules in favor of using make/configure vars

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python: remove --without-ensurepip; not needed anymore
python: cleanup Build/InstallDev rule
python: add HOST_PYTHON_LIB_DIR variable to python-package.mk
python: add PYTHONPATH var; this allows packages to configure it