manifest: container specs added during serialization

Add a second argument, map[string][]container.Spec, during
serialization, which serves the same purpose as the depsolved package
sets.

cmd/osbuild-store-dump/main.go

@@ -36,7 +36,7 @@ func getManifest(bp blueprint.Blueprint, t distro.ImageType, a distro.Arch, d di
 		pkgSpecSets[name] = res
 	}
 
-	mf, err := manifest.Serialize(pkgSpecSets)
+	mf, err := manifest.Serialize(pkgSpecSets, nil)
 	if err != nil {
 		panic(err)
 	}