permit 'third-party' device drivers in other modules

[dellard]

The drivers for individual devices are assumed, by the current QGL compiler, to be installed as a module that is part of QGL itself. In theory, we could have third-party drivers at some point, and it would be somewhat awkward if the only way that the compiler can use a driver is if it is copied into the QGL/drivers directory. It would be nicer if it could be loaded from an arbitrary location, under the direction of the user.