bpo-35713: Reorganize sys module initialization (GH-11658)

* Rename _PySys_BeginInit() to _PySys_InitCore().
* Rename _PySys_EndInit() to _PySys_InitMain().
* Add _PySys_Create(). It calls _PySys_InitCore() which becomes
  private.
* Add _PySys_SetPreliminaryStderr().
* Rename _Py_ReadyTypes() to _PyTypes_Init().
* Misc code cleanup.