Revert ae296b3e6b

This prevents me from running tests, presumably because test projects
& namespaces are not on the classpath. We could add them to the
classpath, but at least at first glance it seems useful to be able to
parse ns-defining code blocks which are not present on the
classpath (such as those in test/marginalia/test/parse.clj).

src/marginalia/parser.clj

@@ -148,7 +148,8 @@
     (if (symbol? sym)
       (do
         (when (= 'ns (first form))
-          (require sym))
+          (try (require sym)
+               (catch Exception _)))
         (let [nspace (find-ns sym)
               docstring (if nspace
                           (-> nspace meta :doc)