[TM-298] Fix lookup of ctor with named field

Description

Problem: when constructor has exectly one field of type :!, CanGoDeeper thinks it should be generically traversed and fails (because there is no instance Generic (:!), and not sure we'd like to define one just for purpose of our lookup).

Solution: halt traversal at :!, because it always corresponds to field.

Related issue(s)

https://issues.serokell.io/issue/TM-298

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