[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.
```
// Note that this might still return null if no main-class is set; we catch that later
```
I'm confused, what might still return null? I don't think this comment is super necessary.