bpo-28411: Remove "modules" field from Py_InterpreterState. (#1638)

sys.modules is the one true source.
diff --git a/Objects/moduleobject.c b/Objects/moduleobject.c
index 02a8cf0..89afe29 100644
--- a/Objects/moduleobject.c
+++ b/Objects/moduleobject.c
@@ -161,11 +161,17 @@
 PyObject *
 PyModule_Create2(struct PyModuleDef* module, int module_api_version)
 {
+    if (!_PyImport_IsInitialized(PyThreadState_GET()->interp))
+        Py_FatalError("Python import machinery not initialized");
+    return _PyModule_CreateInitialized(module, module_api_version);
+}
+
+PyObject *
+_PyModule_CreateInitialized(struct PyModuleDef* module, int module_api_version)
+{
     const char* name;
     PyModuleObject *m;
-    PyInterpreterState *interp = PyThreadState_Get()->interp;
-    if (interp->modules == NULL)
-        Py_FatalError("Python import machinery not initialized");
+
     if (!PyModuleDef_Init(module))
         return NULL;
     name = module->m_name;