This is a temporary solution for now that uses DrawableHitObject.SampleNamespace for the override. We will not want to do this going forward, and instead have the rulesets add their custom resource stores to the games', but that requires deciding where/when to apply/remove such resource stores, and is probably left to skinning.
This is important when using dynamic compiling to rapidly iterate. Until we actually split projects out into pieces (like the abstract ruleset project we have talked about) there is no advantage to using internal in the osu! game code.