This is a problem when syncing a file which just needed its modtime
set with dropbox which can't set the mod time of a file without
re-uploading it.
Before this change we would delete the file, then the server side move
would fail moving the file to the backup-dir because it no longer
existed.
After this change the destination file is moved to the backup-dir
instead of being deleted and the new file is uploaded.
Fixes#2134
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.
