Fix external enums not assignable as constants

- Add external enums test
- Rename external inner class test
- Clean up `GDScriptAnalyzer::reduce_identifier_from_base` class
  behavior

modules/gdscript/tests/scripts/analyzer/features/external_enum_as_constant.gd

@@ -0,0 +1,6 @@
+const External = preload("external_enum_as_constant_external.notest.gd")
+const MyEnum = External.MyEnum
+
+func test():
+    print(MyEnum.WAITING == 0)
+    print(MyEnum.GODOT == 1)

modules/gdscript/tests/scripts/analyzer/features/external_enum_as_constant.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+true
+true

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

@@ -0,0 +1,4 @@
+enum MyEnum {
+    WAITING,
+    GODOT
+}

modules/gdscript/tests/scripts/analyzer/features/external_inner_class_as_constant.gd

@@ -1,4 +1,4 @@
-const External = preload("inner_class_constant_assignment_external.notest.gd")
+const External = preload("external_inner_class_as_constant_external.notest.gd")
 const ExternalInnerClass = External.InnerClass
 
 func test():