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-builder/main.go

@@ -184,7 +184,7 @@ func answer(ctx context.Context, channel *amqp.Channel, publisher builder.Publis
 	fmt.Println()
 
 	built, err := builder.Build(ctx, builder.Command, publisher,
-		request.Repository, request.Path, request.Ref, workspace)
+		request.Repository, request.Path, request.Ref, workspace, request.Held)
 	if err != nil {
 		// A failure is a result. A build that fails and says nothing is indistinguishable from a
 		// builder that is not running, and those want completely different responses.