caos: Remove dubious iteration on Source classes
caos: Remove dubious iteration on Source classes

Currently Source-derived classes expose class pointers iterator via
__iter__, e.g

for ptr_name, ptr_target in Class

Since the need to iterate over class pointers is rare outside Caos core,
it would seem wasteful to allocate class __iter__() for such a purpose.

So, replace calls to __iter__() with an explicit call to
iter_pointers().

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb