[BREAKING API] Add module NCPCore between NCPCompat and NCPPlugin.

* Static utility NCPAPIProvider instead of NoCheatPlus.getAPI().
* Extend NoCheatPlusAPI: Some previously static access methods are now
part of the NoCheatPlusAPI interface instead. MCAccessHolder is
implemented and allows external setting of MCAccess.
* Fix some static members/calls to non-static.
* Moving some packages to NCPCore.
* Prepares for moving most check stuff between NCPCompat and NCPPlugin
to allow more optional higher level components.