MDL-37774 Make moodle1_file_manager::migrate_directory() support trailing slash in rootpath

As the $rootpath is concatenated with the $relpath, we would end with a
path like course_files//file.txt. Such a path is detected as not-valid
later and an error would be thrown.

So the patch makes sure that the trailing slash is cut off and a
debugging for the developer is displayed.
MDL-37774 Do not pass trailing slash if the method does not want it