gh-106320: Remove private AC converter functions (#108505)

Move these private functions to the internal C API
(pycore_abstract.h):

* _Py_convert_optional_to_ssize_t()
* _PyNumber_Index()

Argument Clinic now emits #include "pycore_abstract.h" when these
functions are used.

The parser of the c-analyzer tool now uses a list of files which use
the limited C API, rather than a list of files using the internal C
API.