[entity/url publishing] fetch_rqlst should use 'is_instance_of' instead of 'is'
We usually want to get any subclasses as well (think Training subclass of
Workcase). Closes #3210365.
[rewriter] fix latent bug: arbitrary etype may be substituted when using is_instance_of type restriction
Note: the test is not deterministic because the bug depends on the iteration
order on a set.