Question Not able to implement MRTK interface in runtime script

I am working on my own package and inside one of my scripts inside of the runtime folder, I need to implement the IMixedRealityHandJointHandler interface. This is part of the Microsoft.MixedReality.Toolkit.Input namespace, but it's not able to find it. But if I try to access ArticulatedHandPose for example, which is part of the same namespace, it is able to find that Class. If I move the script outside the package, into my assets folder, it works no problem.