Make skolem definition manager robust to function skolems (#7327)

Make skolem definition manager robust to function skolems (#7327)

This led to a model soundness issue in a development branch that reorganizes theory preprocessing.

This ensures that skolem definition manager looks for skolems that appear in operators.

This is important for inputs like:
(declare-fun h (Int) Int)
(declare-fun f (Int) Int)
(declare-fun g (Int) Int)
(assert (= h (ite A f g)))
(assert (= (h 0) ...)))