Since Moodle 4.0 the course format is a subsystem called core_courseformat. This means that all format plugins can integrate with it using the classes\courseformat, classes\output\courseformat, and templates\courseformat folders.
However, due to historical reasons, even if the format base class is now namespace as core_courseformat\base, the format plugins class is not namespaced and uses a global format_PLUGINNAME one.
In this issue, all format plugins classes will be moved to format/PLUGINNAME/classes/courseformat/format.php. Those new classes will have de format_PLUGINNAME\courseformat namespace and will extend core_courseformant\base class.
There's a proof of concept of the namespaced classes in this commit: