repl: fix referrer for dynamic import

The ESM loader does not accept a directory as the referrer, it requires
a path within the directory.  Add `/repl` to ensure relative dynamic
imports can succeed.

Fixes: https://github.com/nodejs/node/issues/19570

PR-URL: https://github.com/nodejs/node/pull/30609


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Coe <bencoe@gmail.com>