bpo-35081: Internal headers require Py_BUILD_CORE (GH-10363)

* All internal header files now require Py_BUILD_CORE or
  Py_BUILD_CORE_BUILTIN to be defined.
* _json.c is now compiled with Py_BUILD_CORE_BUILTIN to access
  pycore_accu.h header.
* Add an example to Modules/Setup to show how to build _json
  as a built-in module; it requires non trivial compiler options.