repl: fix tab completion of inspector module

Correctly check for the presence of the inspector module before adding
it to the builtin libs list.

PR-URL: https://github.com/nodejs/node/pull/19505


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>