source of `package cl is deprecated'

[Leo Butler]

I updated/upgraded my debian system this morning, which pulled in emacs
27.1.

A number of my home-rolled emacs extensions relied on cl; I have
migrated them to cl-lib, but there is an extension that is still causing
emacs to emit the above-mentioned warning on start-up (and when run with
--daemon, emacs halts and must be killed, which I guess is a bug).

Question: how do I trace/find the offending bit of code? Ideally, I
would like to be able to force an error and use the debugger, but I
can't figure out how to do that.

TIA,
Leo