* actual extension callable, and the part before that refers to the receiver type (either fully
     * or partially qualified).
     *
     * For example, `foo.Foo.bar` may only refer to the extension callable `bar` with
     * a `foo.Foo` receiver type, and this function will only look for such combinations.
     *
     * N.B. This function only searches for extension callables qualified by receiver types!