Use main class from main module if available.

We used to not require the main class, but without checking if there was a "ModuleMainClass" attribute in the main modules module descriptor. As a result of this, if the module did not have a "ModuleMainClass" attribute, the failure would be when the app was run, instead of when we built it. It should be an error to not specify a main class in the --module option unless the modules given has a "ModuleMainClass" attribute.