Fix generation of augmented/partial worlds when single worlds are enabled

- Enabling single worlds adds "PlotSquared:single" to the generators map in SetupUtils
 - Adding a switch for if the map has ever been loaded into means that the first time updateGenerators is called, they will always be updated
 - This means that ultimately PlotSquared is not added as a generator for augmented/partial worlds in bukkit/yml (as it is not, it is a populator)

Core/src/main/java/com/plotsquared/core/util/SetupUtils.java

@@ -34,6 +34,7 @@ import java.util.HashMap;
 public abstract class SetupUtils {
 
     public static HashMap<String, GeneratorWrapper<?>> generators = new HashMap<>();
+    protected boolean loaded = false;
 
     /**
      * @since 6.1.0