deltaic.source: Move into sources subpackage
github: Move team metadata to subdirectory

so no Task's root is nested inside another one.
deltaic.sources: Rename Task to Target

They can also be used to find the storage paths for backup targets, so
we shouldn't just treat them as wrappers around backup subprocesses.
deltaic.sources: Split thread pool out of Source
deltaic.sources: Rename Target to Unit
deltaic.storage: Move mount/umount into Snapshot
Add Task superclass; move Unit.back_up() to _SourceBackupTask