store/compose/ImageType: use distro.ImageType objects

This reduces the amount of resolving and error checking we have to do.
This exposed a bug in weldr's ComposeEntry type, which will be fixed in
a follow-up commit.

Signed-off-by: Tom Gundersen <teg@jklm.no>

cmd/osbuild-composer/main.go

@@ -117,7 +117,7 @@ func main() {
 		logger = log.New(os.Stdout, "", 0)
 	}
 
-	store := store.New(&stateDir)
+	store := store.New(&stateDir, arch)
 
 	queueDir := path.Join(stateDir, "jobs")
 	err = os.Mkdir(queueDir, 0700)