chunked: fix the path used for layers dedup
chunked: fix the path used for layers dedup

`source` is the path to the layer that already contains the file that
is looked up.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

rhatdan/storage,containers/storage,containers/storage,rhatdan/storage,rhatdan/storage,containers/storage