bpo-42260: Initialize time and warnings earlier at startup (GH-23249)

* Call _PyTime_Init() and _PyWarnings_InitState() earlier during the
  Python initialization.
* Inline _PyImportHooks_Init() into _PySys_InitCore().
* The _warnings initialization function no longer call
  _PyWarnings_InitState() to prevent resetting filters_version to 0.
* _PyWarnings_InitState() now returns an int and no longer clear the
  state in case of error (it's done anyway at Python exit).
* Rework init_importlib(), fix refleaks on errors.