mixinOverlayPlan

Resolve what the mixin overlay would contain, or null when there is nothing to build.

jarEntries is the enumeration the shards are about to write, so a target is claimed only if a shard was going to carry it anyway — which also supplies the graft base: the mojmap stub for a Minecraft class, the mod's own class file for everything else. See MixinPlan on why both rename forms are needed.