Rework the dependencies between excuses.
Dependencies are specified based on packages (source or binary). Based
on these packages, dependencies on other excuses (that have these
packages) are calculated.
As with package dependencies, dependencies between excuses can contain
alternatives.
Each alternative can satisfy the dependency, so the excuse only becomes
invalid if all of the alternatives of a specific dependency are
invalidated.
Tracking of the alternatives and their validity is moved to separate
objects.