Right now there's no way to differenciate in sys.path the directories that where added through site.addsitedirs calls, (like when python starts up and initializes site-packages directories)
Anyone can change sys.path, and add elements in them.
So there's no way to answer to this simple question for sure : "what are the site-packages directories ?"
What about keeping track of all directories added through site.addsitedirs calls, with a simple list called "sitedirs"
This list would also override some container methods, so it doesn't keep paths that are not present anymore in sys.path, somehow