Make dynamic classpath modification robust to compiler loader binding

When the current classloader has no modifiable ancestor, do not rely on the
compiler's classloader var binding, as it may be unbound in some circumstances.
Instead, create a new dynamic classloader with the clojure runtime's ancestry.

src/orchard/java/classpath.clj

@@ -72,7 +72,8 @@
   [^URL url]
   (let [loader (or (modifiable-classloader)
                    (set-classloader!
-                    (modifiable-classloader @Compiler/LOADER)))]
+                    (modifiable-classloader (clojure.lang.DynamicClassLoader.
+                                             (clojure.lang.RT/baseLoader)))))]
     (when (dp/add-classpath-url loader url)
       url)))