bpo-32030: Add _PyCoreConfig_Copy() (#4874)

Each interpreter now has its core_config and main_config copy:

* Add _PyCoreConfig_Copy() and _PyMainInterpreterConfig_Copy()
* Move _PyCoreConfig_Read(), _PyCoreConfig_Clear() and
  _PyMainInterpreterConfig_Clear() from Python/pylifecycle.c to
  Modules/main.c
* Fix _Py_InitializeEx_Private(): call _PyCoreConfig_ReadEnv() before
  _Py_InitializeCore()