bpo-30891: Fix importlib _find_and_load() race condition (#2646) (#2651)

* Rewrite importlib _get_module_lock(): it is now responsible to hold
  the imp lock directly.
* _find_and_load() now holds the module lock to check if name is in
  sys.modules to prevent a race condition
(cherry picked from commit 4f9a446f3f)

Python/import.c

@@ -1560,10 +1560,6 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
             if (initializing == -1)
                 PyErr_Clear();
             if (initializing > 0) {
-#ifdef WITH_THREAD
-                _PyImport_AcquireLock();
-#endif
-                /* _bootstrap._lock_unlock_module() releases the import lock */
                 value = _PyObject_CallMethodIdObjArgs(interp->importlib,
                                                 &PyId__lock_unlock_module, abs_name,
                                                 NULL);