`module!`: required keys and ordering

[ojeda]

• Do we want author and description to be required? Find out if there is a reason why new modules should not be always filling those in (samples etc. could perhaps want to skip them, but in general, why would not we want module authors to provide a proper description?).
• Do we want to put all required keys first? The current ordering has license after author and description (because originally they were all required). See previous point too.