Accessing the Libraries of a Project

How do I access the libraries of a Project? (more specifically, how can I programmatically add libraries to a Project?)

I am able to do this if I have a `Module` (get a `ModifiableModel` from it and add a libray to it) but my project doesn't have any modules, so this approach doesn't work, and I can't find any method on the `Project` or `RootProjectManager` object to get such a model.

Thanks!