Detecting functions installed by an extension

Greetings!

How can I tell, using the system catalog or information schema, if a
function/procedure was created by an extension as opposed to by the user
(i.e. would be dropped if you performed "DROP EXTENSION")? So far I've
looked at the `pg_extension` table, which lists the class IDs of
configuration tables but doesn't mention procedures. I've looked at
`pg_proc` but that doesn't seem to contain the information. I've also
scanned over various other system catalogues but with no luck. Is this
information available in one of the system catalogs? Does PostgreSQL itself
track this information so that it can perform cleanup, or does it expect
the extension to clean up after itself?

Thank you for your time,

Benjie.