Split dockerImageDestination.blobExists from dockerImageDestination.TryReusingBlob
Split dockerImageDestination.blobExists from dockerImageDestination.TryReusingBlob

We will add more calls to blobExists from TryReusingBlob soon.

Should not change behavior.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image