Can't iterate easily on plugin policies

It is difficult to iterate on plugin development. When adding a plugin with a policy into apiman, there really isn't a way to fully delete it. The policy definition contributed by the plugin is discovered and added to the policy definitions table - and then can never be removed.

Overall, we need to be able to more easily add/remove plugins and have their contributed policy definitions automatically removed.

Perhaps we can simplay hide/disable policy definitions from plugins when a plugin is deleted.

What do we do if the policy definition is referenced by a policy instance? And what if that policy definition refers to a plugin resource?

Do we only handle iterating on -SNAPSHOT plugins?