Hi,
You can now use a PSR-4 autoloader[1] in MediaWiki core and via extension.json.
If you're not familiar with PSR-4, it's a standard which tells autoloaders that if a class is named Foo\Bar\Baz, it can be found at $basePath/Bar/Baz.php. This means that we don't need to keep a mapping of classes to filenames, and instead just need to register the namespace with the autoloader.
In extension.json, you can register PSR-4 compliant namespaces with the "AutoloadNamespaces" property[2]. You can also look at the ContentTranslation, Linter, or ORES extensions for examples.
[1] http://www.php-fig.org/psr/psr-4/ [2] https://www.mediawiki.org/wiki/Manual:Extension.json/Schema#AutoloadNamespac...
-- Kunal / Legoktm