Part pkg Mixin Refactor

Description

I'm looking at refactoring parts of the part_pkg architecture. The previous refactor was great, in that it made it easy to create new plan types, based on perl modules. The current problem that I see is there is only a linear inheritance. What I propose is to move to a Mixin style architecture, as many of the other parts of Freeside have done. This would allow us to create basic code for concepts such as 'Introrate' or 'Referrals' which could quickly be added to any package type, not copied and pasted into new sets.