Moveable packages dir

This is a feature request basically so we can deny it if needed for 2.1.  I am proposing we add support in SMF to be able to move the packages directory.  I am accomplishing this by adding a new Settings.php variable, having upgrade.php add that and updated all revelvant parts of SMF to use that directory.

I've done some testing and haven't seen anything broken or bad side effects from it yet.

Signed off by me