Job-level definitions should not depend on task-related modules.

The two concepts are tightly coupled and it does not make sense
to mix and match them. Therefore, it's cleaner and easier to
understand if both aspects of the preemption model are instantiated
in the same module next to each other.

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.