bpo-40549: Convert posixmodule.c to multiphase init (GH-19982)

Convert posixmodule.c ("posix" or "nt" module) to the multiphase
initialization (PEP 489).

* Create the module using PyModuleDef_Init().
* Create ScandirIteratorType and DirEntryType with the new
  PyType_FromModuleAndSpec() (PEP 573)
* Get the module state from ScandirIteratorType and DirEntryType with
  the new PyType_GetModule() (PEP 573)
* Pass module to functions which access the module state.
* convert_sched_param() gets a new module parameter. It is now called
  directly since Argument Clinic doesn't support passing the module
  to an argument converter callback.
* Remove _posixstate_global macro.