Overhaul Resource::duplicate_for_local_scene()

- Serves as a first step for future refactors.
- Code is simpler.
- Algorithm is more efficient: instead of two passes (dumb copy + resolve copies), it's single-pass.
- Now obeys `PROPERTY_USAGE_NEVER_DUPLICATE`.
- Now handles deep self-references (the resource to be duplicated being referenced somewhere deep).

core/io/resource.h

@@ -83,7 +83,7 @@ private:
 
 	SelfList<Resource> remapped_list;
 
-	void _dupe_sub_resources(Variant &r_variant, Node *p_for_scene, HashMap<Ref<Resource>, Ref<Resource>> &p_remap_cache);
+	Variant _duplicate_recursive_for_local_scene(const Variant &p_variant, Node *p_for_scene, HashMap<Ref<Resource>, Ref<Resource>> &p_remap_cache);
 	void _find_sub_resources(const Variant &p_variant, HashSet<Ref<Resource>> &p_resources_found);
 
 protected: