Duplicate (:refer-clojure) parse map with 'cljs.core

In ClojureScript, cljs.core defines macros for the clojure.core
namespace, and is mass-referred into namespaces by default.

Therefore, (:refer-clojure :exclude […]) should also :exclude vars
within 'cljs.core.

e.g.
        (ns my.ns.macros
          (:refer-clojure :exclude [defn]))

        (defmacro defn [] …)

        (ns my.ns
          (:refer-clojure :exclude [defn]))

        (defn foo [])

Reconstructing my.ns would likely add '{:refer {cljs.core #{defn}}} as a
candidate since cljs.core is shorter than my.ns.macros.