How to obtain a list of the submodules of a module using Drush?
This information is available via the pm-projectinfo command:

$ drush pm-projectinfo views
 Name         :  Views (views) 
 Version      :  7.x-3.13      
 Extensions   :  views         
                 views_test    
                 views_ui     


This command is available in Drush 8, and was first introduced in this PR.  It is also available in Drush Extras for older versions of Drush.