Fix GDScript base and outer classes, signals and functions lookup order

- Add outer class lookup test
- Add signal lookup test

Co-authored-by: Dmitrii Maganov <vonagam@gmail.com>

modules/gdscript/tests/scripts/analyzer/features/lookup_class_external.notest.gd

@@ -0,0 +1,15 @@
+class A:
+	const TARGET: = "wrong"
+
+	class B:
+		const TARGET: = "wrong"
+		const WAITING: = "godot"
+
+		class D extends C:
+			pass
+
+class C:
+	const TARGET: = "right"
+
+class E extends A.B.D:
+	pass