Say why assign finds no module, and keep an assignment pending on its build

A merge asks for a new module's build, and assign answered "no module of that
name" until the build registered it, which read as a module nobody registered
(hq issue 325). Keep every build request, tell a build in flight, a module
known and not built, and an unknown name apart, and make an assignment made
while the build runs when the build registers the module.

cmd/mesh-controller/modules.go

@@ -354,9 +354,20 @@ func moduleCommand(ctx context.Context, args []string) error {
 func assignCommand(ctx context.Context, verb string, args []string) error {
 	// Several modules in one act (novox/hq ADR 0207): holders that depend on each other — the
 	// service manager and the package manager — can only go on, or come off, together.
+	set := flag.NewFlagSet(verb, flag.ContinueOnError)
+	// A module known and not built: ask for its build, and keep the assignment pending on it (novox/hq
+	// issue 325). Only assign reads it.
+	build := set.Bool("build", false, "for a module known and not built: ask for its build, and assign it when it registers")
+	args, err := parseAround(set, args)
+	if err != nil {
+		return err
+	}
 	if len(args) < 2 {
 		return fmt.Errorf("%s <node> <module> [<module>…]", verb)
 	}
+	if *build && verb == "unassign" {
+		return errors.New("unassign takes no --build")
+	}
 	open, err := openStores(ctx)
 	if err != nil {
 		return err
@@ -365,7 +376,9 @@ func assignCommand(ctx context.Context, verb string, args []string) error {
 
 	// The act itself is in acts.go, so the command API refuses exactly what this refuses
 	// (novox/hq ADR 0035). What differs between the surfaces is how the answer is printed.
-	act := assign
+	act := func(ctx context.Context, open *stores, node string, modules ...string) (string, error) {
+		return assignWith(ctx, open, node, assignOptions{Build: *build}, modules...)
+	}
 	if verb == "unassign" {
 		act = unassign
 	}