Remove the now-unused InstallPlan type args for result and failure
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.