Use Drupal's autoload.php instead of Composer's.
Fork Composer's ClassLoader into Drupal\Core\Autoload\ClassLoader
Switch our ClassLoader fork to Composer's PSR-4 ClassLoader as proposed in https://github.com/composer/composer/pull/2121/files
Register modules as PSR-4 instead of PSR-0, without moving the classes. Allow for multiple paths per prefix.
Register core namespaces as PSR-4
Use a loader with additional predictor character for PSR-4. Leave PSR-0 unchanged.
Register test namespaces as PSR-4.