bpo-35713: Split _Py_InitializeCore into subfunctions (GH-11650)

* Split _Py_InitializeCore_impl() into subfunctions: add multiple pycore_init_xxx() functions
* Preliminary sys.stderr is now set earlier to get an usable
  sys.stderr ealier.
* Move code into _Py_Initialize_ReconfigureCore() to be able to call
  it from _Py_InitializeCore().
* Split _PyExc_Init(): create a new _PyBuiltins_AddExceptions()
  function.
* Call _PyExc_Init() earlier in _Py_InitializeCore_impl()
  and new_interpreter() to get working exceptions earlier.
* _Py_ReadyTypes() now returns _PyInitError rather than calling
  Py_FatalError().
* Misc code cleanup