gh-106320: Remove private _PyLong_FileDescriptor_Converter() (#108503)

Move the private _PyLong converter functions to the internal C API

* _PyLong_FileDescriptor_Converter(): moved to pycore_fileutils.h
* _PyLong_Size_t_Converter(): moved to pycore_long.h

Argument Clinic now emits includes for pycore_fileutils.h and
pycore_long.h when these functions are used.