Should we deprecate Kernel::getRootDir()? With the new structure and Kernel living in src/ this will basically point to some arbitrary namespace path. Im not even sure core still uses/needs it, having getProjectDir() also.

I think also any dirname(__DIR__) call in https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/3.3/src/Kernel.php should be replaced by that (getProjectDir).