The Prosa notion of priority-driven scheduling depends on the
preemption model. That is ok; any theory that doesn't want to deal
with non-preemptive sections can simply Require Export the fully
preemptive model and be done with it.

With this patch, the model module is finally completely independent of
any definitions or proofs in the analysis module.

This fixes all warnings about ambiguous module names and resolves #49.

It also highlights that we still need to cut down on superfluous
Require Import commands in recently ported files.