See change I5574895b3cabc46ca74f67113de1a03959e2abf3: "Autodiscover classfiles via namespace hierarchy", and https://github.com/php-fig/fig-standards/blob/master/proposed/psr-4-autoloader/psr-4-autolader.md
Offer an additional autoloader mechanism.
I'm not sure if this is the top-level task, but see T85182. We've implemented autoloading using static classmaps which are generated by a script, composer autoload. This fixes the most annoying failure to DRY, since we no longer have to add newly created classes to the crazy-long master list.