[DI][Routing] recursive directory loading

Issue #11045

For now, the Routing DirectoryLoader requires the type `directory`
to be specified so it does not conflict with `AnnotationDirectoryLoader`.
However, this could be refactored.

The type declaration requires a [patch in AsseticBundle](https://github.com/symfony/AsseticBundle/commit/f76291c4655f6b7b29f4d20650c020d3be516ad5)
for it to be used on `framework.router.resource`. The patch is already merged in master, but not in a release yet.