Load comm targets and widget models from requirejs modules

Ping @jdfreder @jasongrout @SylvainCorlay

This continues my work in #6494, where I did the same with widget views. These other parts were more straightforward.

I've not actually tested this with targets/models in modules, as I don't have a handy example. I have tested that instantiating a widget using the target & model registries still works.