I found a work-around. I have to define my module using mxml instead of straight ActionScript. For some reason it wasn't loading the halo skin classes into the module and since they weren't available in the main app either it would crash when it tried to get the class name from the default css file and instantiate it.
So basically an ActionScript object which extends Module is not sufficient for creating a module which contains certain components which are skinned using skin classes. Its kind of dumb in my case because I don't want any of my objects to have parents (they will be placed in the main application view once loaded). I have to use the initialize="onInitialize()" event to create my objects instead of a constructor.
