Move Lib\Backend to PSR-4

apps/files_external/lib/Lib/Backend/AmazonS3.php

apps/files_external/lib/Lib/Backend/Backend.php

apps/files_external/lib/Lib/Backend/DAV.php

apps/files_external/lib/Lib/Backend/Dropbox.php

apps/files_external/lib/Lib/Backend/FTP.php

apps/files_external/lib/Lib/Backend/Google.php

apps/files_external/lib/Lib/Backend/LegacyBackend.php

apps/files_external/lib/Lib/Backend/Local.php

apps/files_external/lib/Lib/Backend/OwnCloud.php

apps/files_external/lib/Lib/Backend/SFTP.php

apps/files_external/lib/Lib/Backend/SFTP_Key.php

apps/files_external/lib/Lib/Backend/SMB.php

apps/files_external/lib/Lib/Backend/SMB_OC.php

apps/files_external/lib/Lib/Backend/Swift.php

apps/files_external/tests/Backend/BackendTest.php

apps/files_external/tests/Backend/LegacyBackendTest.php