As of Drupal 8, each directory can only contain a maximum of one module, in order to better support PSR-4 and the configuration system.
While in Drupal 7, it was acceptable to have a module directory structure like:
sites/all/modules/views/views.module .. sites/all/modules/views/views_ui.module
In Drupal 8, it should be:
sites/all/modules/views/views.module ... sites/all/modules/views/views_ui/views_ui.module
Note that custom and contrib projects may also be placed in new top-level 'modules', 'themes', and 'profiles' directories. See the change record at http://drupal.org/node/1766160 for more information.