Make loading service extensions work with classloader path

Fixed: https://github.com/jruby/jruby/issues/2055

Note that the logic added to JarredScript is almost identical to logic in LibrarySearcher when dealing with loading a jar file (https://github.com/jruby/jruby/blob/jruby-1_7/core/src/main/java/org/jruby/runtime/load/LibrarySearcher.java#L309)

It may make sense to see if the logic can be combined. On other hand, since mkristian is pretty close to killing findLibraryWithClassloaders in master, maybe the effort is not necessary.