Use an internal type for installing Python modules

When this is set, the version is used to set the variable. If the
version changes in the future without nuking the variable, it is invalid
(still pointing at the original version). Since users probably shouldn't
be messing with it anyways, just make it internal.

Change-Id: I544f353809f07bf7e62c1c355a901c222898e0ca
Merge topic 'fix-python-module-install-dir' into next

643c9d3 Use an internal type for installing Python modules