gh-81057: Move PyImport_Inittab to _PyRuntimeState (gh-99402)

We actually don't move PyImport_Inittab.  Instead, we make a copy that we keep on _PyRuntimeState and use only that after Py_Initialize().  We also prevent folks from modifying PyImport_Inittab (the best we can) after that point.

https://github.com/python/cpython/issues/81057

Python/pylifecycle.c

@@ -605,6 +605,11 @@ pycore_init_runtime(_PyRuntimeState *runtime,
         return status;
     }
 
+    status = _PyImport_Init();
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     status = _PyInterpreterState_Enable(runtime);
     if (_PyStatus_EXCEPTION(status)) {
         return status;