[FIXES  JENKINS-18528]: load plugin scripts via CLASSPATH if available

Starting from Jenkins 1.519, it deploys plugins as single `WEB-INF/lib/classes.jar` file (before 1.519, it deploys plugin as directory of `WEB-INF/classes`). This patch will load plugin scripts via ClassLoader if available. For backward compatibility, if there is exploded plugin directory, it loads plugin scripts from directory.