API to determine plugin providing a certain class

[A248]

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It would be useful for Velocity to provide a method that allows determining which plugin is providing a certain class. This is helpful especially in debugging classpath conflicts.

The Bukkit API provides this functionality with JavaPlugin.getProvidingPlugin, though I of course trust the Velocity API to do better than a static method on JavaPlugin. A new method in PluginManager would do well for the Velocity API.

[Xernium]

Depending on how we handle things going forward in Polymer, this will end up being an added feature anyway for Plugin space capsulation to work