Script to move class files to their new PSR-4 location

Follow-up to
#1971198: [policy] Drupal and PSR-0/PSR-4 Class Loading
#2033611: Port AnnotatedClassDiscovery to PSR-4, and make it agnostic of directory information.
#2058867: Consider introducing a native ClassLoader for performance and PSR-4

We want a script to easily move all module class files from their PSR-0 location to their new PSR-4 location.

This already existed in the Krautoload patch, so we can just pick it up from there.