geiser-xref-callers or geiser-xref-callees doesn't seem to work

Hello!

For as long as I've been using Geiser with Guile (roughly, 6 years), I don't think I've ever seen geiser-xref-callers or geiser-xref-callees return any result. Granted, I don't try very often :-).

I thought I'd finally report the issue.

I typically work with the Guix source code but I think it was not working either in smaller code bases such that of GNU mcron.

Thank you for Geiser!