gh-118527: Use _Py_ID(__main__) for main module name (#118528)

Most module names are interned and immortalized, but the main
module was not. This partially addresses a scaling bottleneck in the
free-threaded when creating closure concurrently in the main module.

Python/pylifecycle.c

@@ -2348,7 +2348,7 @@ static PyStatus
 add_main_module(PyInterpreterState *interp)
 {
     PyObject *m, *d, *ann_dict;
-    m = PyImport_AddModule("__main__");
+    m = PyImport_AddModuleObject(&_Py_ID(__main__));
     if (m == NULL)
         return _PyStatus_ERR("can't create __main__ module");