lang/python27(3*), Mk/Uses/python.mk: remove make spawn

There are some inefficiencies in python.mk that significantly slow down
full tree scanning. The use of bmake to obtain the current version of
a specific python is responsible for the majority of the slow done.

This commit splits out the PYTHON_PORTVERSION definition (which is the
same as the lang/python* PORTVERSION) into separate files. With this
change, python.mk can simple include the makefile fragment instead of
spawning a new instance of make.

Different Revision: https://reviews.freebsd.org/D4660
Approved by: antoine (python), mva (python)