Detect if images are available if we don't have to generate a Package

While building, if we aren't doing a clean build, we scan now to see if
images are available and we skip, in case we don't find a metadata
already.

pkg/compiler/backend/simpleimg.go

@@ -97,6 +97,10 @@ func (*SimpleImg) CopyImage(src, dst string) error {
 	return nil
 }
 
+func (*SimpleImg) ImageAvailable(imagename string) bool {
+	return imageAvailable(imagename)
+}
+
 func (*SimpleImg) ImageExists(imagename string) bool {
 	// NOOP: not implemented
 	// TODO: Since img doesn't have an inspect command,