When sorting fs.DirEntries we sort by DirEntry type and
when synchronizing files let the directories be before objects,
so when the destintation fs doesn't support duplicate names,
we will only lose duplicated object instead of whole directory.
The enables synchronisation to work with a file and a directory of the same name
which is reasonably common on bucket based remotes.
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.
