[SPARK-4298][Core] - The spark-submit cannot read Main-Class from Manifest.

Resolves a bug where the `Main-Class` from a .jar file wasn't being read in properly. This was caused by the fact that the `primaryResource` object was a URI and needed to be normalized through a call to `.getPath` before it could be passed into the `JarFile` object.
Minor nit, but this comment actually applies to the `mainClass = jar.getManifest.getMainAttributes.getValue("Main-Class")` line, so it would be best to keep it next to that line instead of moving it here.