Following the addition of PSR-4 support to Drupal 8, the next step will be to move all Drupal 8 core module code to PSR-4 (Issue #2247991).

Since this change is significant and will require the reroll of many existing patches, a potential commit window for is scheduled for May 27, to give contributors a chance to prepare for this change and update their patches before DrupalCon Austin. Drupal 8.0-alpha12 will now be released on May 28 to allow time for this change. (If the patch for Issue #2247991 is not ready by May 27, it will be postponed until after DrupalCon.)

After this change, the deprecated PSR-0 directory structure will continue to work for contributed modules, but this support will be dropped before the first beta in Issue #2247287, so you should also update your Drupal 8 contributed modules to PSR-4.