Read signatures from UnparsedImage instead of ImageSource directly

This should not make a difference right now, because nothing else
uses the UnparsedImage, but it allows caching _in principle_
and it makes the code more similar to the copyOneImage version,
which we'll benefit from next.

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

copy/copy.go

@@ -401,7 +401,7 @@ func (c *copier) copyMultipleImages(ctx context.Context, policyContext *signatur
 		sigs = [][]byte{}
 	} else {
 		c.Printf("Getting image list signatures\n")
-		s, err := c.rawSource.GetSignatures(ctx, nil)
+		s, err := unparsedToplevel.Signatures(ctx)
 		if err != nil {
 			return nil, perrors.Wrap(err, "reading signatures")
 		}