Don't nullify module when has_mfs

setuptools/namespaces.py

@@ -51,7 +51,7 @@ class Installer:
                 "importlib.util.module_from_spec("
                     "importlib.machinery.PathFinder.find_spec(%(pkg)r, "
                         "[os.path.dirname(p)])))",
-        "m = not has_mfs and "
+        "m = m or not has_mfs and "
             "sys.modules.setdefault(%(pkg)r, types.ModuleType(%(pkg)r))",
         "mp = (m or []) and m.__dict__.setdefault('__path__',[])",
         "(p not in mp) and mp.append(p)",