Some complex re-ordering to make prebuild do what needed to be done.

It is now possible to use module interfaces without referencing Scene.
Place those interfaces in OpenSim/Region/Interfaces. They may not
use any refs from OpenSim.Region.Environment as parameters.
This resolves a circular library ref introduced in r5949

OpenSim/Region/Environment/Scenes/SceneBase.cs

@@ -220,5 +220,10 @@ namespace OpenSim.Region.Environment.Scenes
 
             return null;
         }
+
+        public virtual T RequestModuleInterface<T>()
+        {
+            return default(T);
+        }
     }
 }