FromClasspathEntryClassInformationProvider too eager to verify MainClass

FromClasspathEntryClassInformationProvider is attempting to verify (eagerly) whether the given MainClass is on the user classpath. However, it doesn't handle cases where the main class is inside a nested jar.

We actually don't need this check at all since PackagedProgram is already doing it while attempting to load the main class. Having this once should be enough.