Fix duplicate namespaces

Author: @martinklepsch

When there are multiple files (e.g. .clj and .cljc) for a namespace
Codox would analyse those namespaces twice returning the analysis
result twice as well.

cljdoc@3f2ea84
cljdoc/cljdoc#155

codox/src/codox/reader/clojure.clj

@@ -116,8 +116,8 @@
 
 (defn- find-namespaces [file]
   (cond
-    (.isDirectory file) (ns/find-namespaces-in-dir file)
-    (jar-file? file)    (ns/find-namespaces-in-jarfile (JarFile. file))))
+    (.isDirectory file) (set (ns/find-namespaces-in-dir file))
+    (jar-file? file)    (set (ns/find-namespaces-in-jarfile (JarFile. file)))))
 
 (defn read-namespaces
   "Read Clojure namespaces from a set of source directories (defaults