Fix readOnly rule to handle OpenAPI 2 polymorphism pattern.

[mikekistler]

The readOnly rule categorizes schemas as "request" or "response" if they are reachable from an operation request or response body, respectively. But the current logic does not handle the OpenAPI 2 polymorphism pattern, where the parent schema does not reference child schemas directly -- instead, the child schema "allOf"s the parent into itself. As a result, the rule is generating false positives, such as in this PR.