Stubtest: clean up the _belongs_to_runtime function (#14361)

There's a small semantic change in this PR (instead of returning `False`
if trying to access the `__module__` attribute raises an exception, we
now just move on to the next heuristic). But the main purpose of this PR
is to make the code more readable, as this function was getting quite
hard to understand.

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

mypy/stubtest.py

@@ -346,16 +346,20 @@ def verify_mypyfile(
     imported_symbols = _get_imported_symbol_names(runtime)
 
     def _belongs_to_runtime(r: types.ModuleType, attr: str) -> bool:
+        """Heuristics to determine whether a name originates from another module."""
         obj = getattr(r, attr)
         if isinstance(obj, types.ModuleType):
             return False
         if callable(obj):
+            # It's highly likely to be a class or a function if it's callable,
+            # so the __module__ attribute will give a good indication of which module it comes from
             try:
-                obj_mod = getattr(obj, "__module__", None)
+                obj_mod = obj.__module__
             except Exception:
-                return False
-            if obj_mod is not None:
-                return bool(obj_mod == r.__name__)
+                pass
+            else:
+                if isinstance(obj_mod, str):
+                    return bool(obj_mod == r.__name__)
         if imported_symbols is not None:
             return attr not in imported_symbols
         return True
@@ -367,8 +371,9 @@ def _belongs_to_runtime(r: types.ModuleType, attr: str) -> bool:
             m
             for m in dir(runtime)
             if not is_probably_private(m)
-            # Ensure that the object's module is `runtime`, since in the absence of __all__ we
-            # don't have a good way to detect re-exports at runtime.
+            # Filter out objects that originate from other modules (best effort). Note that in the
+            # absence of __all__, we don't have a way to detect explicit / intentional re-exports
+            # at runtime
             and _belongs_to_runtime(runtime, m)
         }
     )