List or ListR of an non existent directory must return
ErrorDirNotFound for non bucket based remotes. For bucket based
remotes it may return ErrorDirNotFound or it may return no error and
no entries.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
