Register a module at artifacts the store holds, by hand, recorded as a build and a hand act, so the controller can be delivered without the builder (issue 1888, C8a and C8b)

internal/artifacts/store.go

@@ -291,3 +291,32 @@ func (s Store) HoldsManifest(ctx context.Context, reference string) (bool, error
 	}
 	return s.has(ctx, s.url(repository, kind, digest), anyManifest...)
 }
+
+// Holds is whether the store holds what a recorded reference names: the blob of an archive, the manifest of an image.
+// Gone when it does not; asks and changes nothing (ADR 0526 clause 3: a registration by hand is refused at bytes that
+// are not there).
+func (s Store) Holds(ctx context.Context, reference string) error {
+	path, ours := catalogue.InArtifactStore(reference)
+	if !ours {
+		return fmt.Errorf("%w: %s", ErrNotOurs, reference)
+	}
+	repository, kind, digest, err := split(path)
+	if err != nil {
+		return err
+	}
+	if s.Address == "" {
+		return fmt.Errorf("this mesh has no artifact store on its network to ask about %s", reference)
+	}
+	if kind == "blobs" {
+		_, err := s.blobSize(ctx, repository, digest)
+		return err
+	}
+	held, err := s.has(ctx, s.url(repository, kind, digest), anyManifest...)
+	if err != nil {
+		return err
+	}
+	if !held {
+		return Gone
+	}
+	return nil
+}