Fix Misc.urlExists for subdirectories in Jar files

Currently, `Misc.urlExists` attempts to open a stream to check for existence of a URL. This doesn't make sense when looking for a subdirectory, and breaks for subdirectories in Jar files due to a bug in the JRE.

This update uses `URLClassLoader.findResource` to check for the existence of URLs located within Jar files, which works for both files and folders within Jar files.