Only select dependencies in the pg_class catalog

[MikeN123]

This makes sure we do not select dependencies form the pg_operator class or other similar things.
Otherwise, if your FK has a dependency on something like a pg_operator (e.g., when your FK is pointing to a CITEXT column), things will break with the message:

ERROR pgsql: The value
         NIL
       is not of type
         PGLOADER.CATALOG:INDEX

during PGLOADER.PGSQL:LIST-ALL-FKEYS.

I'm not an expert on the exact contents on the pg_class and pg_depend tables, so can't guarantee this is the 100% correct fix.

Fixes #1289