This pr reverts a change made in 2012 which allowed CraftItem to be a also other entities such as an arrow.
I don't see any practical reason why the original change was done, other than to allow plugin developers to do some stuff in a cursed way.
With the introduction of the PlayerPickupArrowEvent in 2016 there is also a way to access the arrow involved in a better way.
Generally I think the current behavior is a bit strange and should be reverted.
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