Use ext: instead of si: on Embeddable Common-Lisp

The system package (si for short) is internal and "never to be used"
according to https://common-lisp.net/project/ecl/manual/ch11.html

This change does not require testing because (presumably
unintentionally) the process-info-pid function generates an error for
ECL anyway (to be fixed in a follow-up commit)