repl: move completion logic to internal module

PR-URL: #59889
Backport-PR-URL: #61046
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

Author: targos