baseVars/hostVars: use module system instead of specialArgs

[llakala]

Closes #13.

Could be made a bit prettier with:

• Creating a myLib function for automatic isDefined assertions
• Looking into the possibility of aliasing config.hostVars to just hostVars.
• moving configDirectory to be a host variable since if we ever changed it on one machine, it shouldn't break other systems
• importing baseVars and hostVars automatically via a fancy mkNixos helper function
• Thinking about paradigms that let us access vars regardless of whether they're a hostVar or a baseVar.

but that can all come in a later commit. For now, this adds the functionality, which is much better than the previous cursed specialArgs stuff.