Leave an eligible index for a confirmed collect instead of letting it go alone

Let go of alone after a build, an index's platforms stayed for ever under a record that
said collected, so no later collect could reach them (re-review of #144).

internal/artifacts/store.go

@@ -261,6 +261,21 @@ func split(path string) (repository, kind, digest string, err error) {
 	return "", "", "", fmt.Errorf("%w: %q names nothing the store holds by digest", ErrNotOurs, path)
 }
 
+// IsIndex is whether a recorded reference is an index the store holds: one that names manifests. A
+// blob, or a manifest that names none, is not. Gone when the store does not hold it.
+func (s Store) IsIndex(ctx context.Context, reference string) (bool, error) {
+	path, ours := catalogue.InArtifactStore(reference)
+	if !ours {
+		return false, fmt.Errorf("%w: %s", ErrNotOurs, reference)
+	}
+	repository, kind, digest, err := split(path)
+	if err != nil || kind != "manifests" {
+		return false, err
+	}
+	children, err := s.Platforms(ctx, repository, digest)
+	return len(children) > 0, err
+}
+
 // HoldsManifest is whether the store holds the manifest a recorded image reference names.
 func (s Store) HoldsManifest(ctx context.Context, reference string) (bool, error) {
 	path, ours := catalogue.InArtifactStore(reference)