Service providers, and other global API-like elements, separated so dependencies don't need to be on the core forge system.

• net.minecraftforge.api.distmarker.* are the Dist/OnlyIn annotation pair, used to identify elements that are only present in one of the two common distributions.
• net.minecraftforge.forgespi.ICoreMod* are interfaces for communication between Forge and the coremod library.