A module names the module its build stands on, not a copy of it

A fingerprint written into a recipe names one particular copy of the base — the
copy on whichever machine the person typing it was using. On any other mesh that
copy has never existed, so the build stops on its first line with a message
about an image nobody can look up. Three modules in the catalogue were in
exactly that state, and the line each of them replaced was equally dead.

A module now names the module and artifact instead, and the mesh answers with
what it holds. The builder is still a thing that clones, builds and answers: the
answer travels with the question, because only the mesh knows what it has.

A base the mesh has not built is refused before anything is built, naming which
module has to exist first.

cmd/mesh-control/build.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"os"
 	"strings"
 	"time"
 
@@ -341,6 +342,7 @@ func buildOne(ctx context.Context, repository, path, ref string, wait time.Durat
 		Repository: repository,
 		Path:       path,
 		Ref:        ref,
+		Held:       heldBy(ctx),
 	}
 	fmt.Printf("asked for %s", request.Repository)
 	if path != "" {
@@ -417,6 +419,7 @@ func buildAndShow(ctx context.Context, repository, path, ref string, wait time.D
 	result, err := link.RequestBuild(ctx, server.Channel(), link.BuildRequest{
 		ID:         fmt.Sprintf("%s-%d", "build", time.Now().UnixNano()),
 		Repository: repository, Path: path, Ref: ref,
+		Held: heldBy(ctx),
 	}, wait)
 	if err != nil {
 		return err
@@ -460,3 +463,25 @@ type answers struct {
 	// a mesh whose hub is that node has no hub.
 	network string
 }
+
+// heldBy is every artifact this mesh has built, for a build that may need one as its base.
+//
+// **A failure here is not a failure to build.** A module that names no base does not need this at
+// all, and one that does gets a refusal naming exactly what is missing — which is a better sentence
+// than a build command refusing to start because a query did not run. So the store not opening is
+// reported and the build goes ahead without it.
+func heldBy(ctx context.Context) map[string]string {
+	open, err := openStores(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not read what this mesh has built, so a module naming a "+
+			"base will be told that base is missing: %v\n", err)
+		return nil
+	}
+	defer open.Close()
+	held, err := open.inventory.Held(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not read what this mesh has built: %v\n", err)
+		return nil
+	}
+	return held
+}