Add a dedicated option for "include system site-packages" rather than listing it in bases
This would solve issue #1 and issue
#2, because we'd no longer need to find pythonv.conf in the global Python install. It also seems a little redundant to have to specify the full system Python prefix in bases when it can be derived from the Python binary we specify in python=.
The challenge would be the inheritance semantics, because assuming this is a true/false config, I think it should be an OR of all the bases' values rather then preferring the most local. If any virtual environment in bases specifies to include the system packages, they should be included.