Provide a fullname for __module__ (#20288)

Fixes #20284

---------

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

Author: A5rocks

mypy/semanal.py

@@ -6511,7 +6511,9 @@ def _lookup(
                 implicit_node = node
         # 2b. Class attributes __qualname__ and __module__
         if self.type and not self.is_func_scope() and name in {"__qualname__", "__module__"}:
-            return SymbolTableNode(MDEF, Var(name, self.str_type()))
+            v = Var(name, self.str_type())
+            v._fullname = self.qualified_name(name)
+            return SymbolTableNode(MDEF, v)
         # 3. Local (function) scopes
         for table in reversed(self.locals):
             if table is not None and name in table:

test-data/unit/check-classes.test

@@ -8927,6 +8927,11 @@ class C:
         __module__  # E: Name "__module__" is not defined
         __qualname__  # E: Name "__qualname__" is not defined
 
+    x: __module__  # E: Variable "__main__.C.__module__" is not valid as a type \
+                   # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+    y: __qualname__  # E: Variable "__main__.C.__qualname__" is not valid as a type \
+                     # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+
 [case testPropertySetterType]
 class A:
     @property