These were used previously for the Installed and Failed package states,
but these states are now gone.

Importantly this now means that we can have a serialisable InstallPlan
without the failure types having to be serialisable. This means we can
use things like SomeException which is not serialisable. Since the
traversal is done separately, the result of the traversal contains the
failure values, but this result set does not have to be serialised.