Split source sets

[Technici4n]

Feature request: Introduce split source sets, a feature that mods can opt into to have client and common code in separate source sets. This provides a compile-time guarantee that client code will never be called from common code (which also needs to run on dedicated servers). It has been confirmed that Minecraft is developed with a similar setup.

Potential problems:

• The common source set should not be allowed to call client-only code from other mods. One solution is to auto-split mod dependencies based on the @OnlyIn annotation (requires mod dependency scanning :/). Another solution is to allow mods to publish separate artifacts for their client-only and common code.
• Split source sets need to be used in NeoForge before they are usable in userdev.
• (To be confirmed) Eclipse support for source sets is quite bad, and might not support this.

[lukebemish]

Allowing mods to publish separate client/shared artifacts should not be particularly difficult, all things considered; it's not hard to solve at all with the variant selection tools that gradle gives you.