Reorganized artifact-related classes

- files now generally contain only 1 class (except for tightly coupled
classes)
- files are now located in lib/entities/artifact directory
- removed excessive includes

No changes to functionality

lib/CCreatureSet.cpp

@@ -10,7 +10,6 @@
 #include "StdInc.h"
 #include "CCreatureSet.h"
 
-#include "ArtifactUtils.h"
 #include "CConfigHandler.h"
 #include "CCreatureHandler.h"
 #include "GameLibrary.h"