One possible solution to mutual definitions is to refactor the individual classes, and try to abstract the prerequisites into "parent" classes.

What does that mean? Well, basically you pull out the functions needed by both packages, and put them into a parent package. You might need to do this for a number of layers to completely decouple the mutual requirements. Essentially, it's akin to many-to-many relationships in databases, which are frequently resolved by using aggregated relationships.