Pin down Derived constraints

[nfrisby]

I still don't feel confident about anticipating when, where, and why Derived constraints arise. For now, simplifyWanteds2 ignores them, and I'm pretty sure that's wrong.

A large part of the work for this task will be establishing a solid understanding of when/where/why Derived constraints arise. After we have that, I think adjusting the plugin won't be so much work.