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/seatverbs.go

@@ -418,7 +418,13 @@ func (a *verbArguments) commandLine() ([]string, error) {
 		}
 		// Several modules comma-separated, judged as one act (novox/hq ADR 0207): the holders of
 		// the seats that apply resources depend on each other and go on together.
-		return append([]string{verb, str("node")}, splitModules(str("module"))...), nil
+		argv := append([]string{verb, str("node")}, splitModules(str("module"))...)
+		// A module known and not built: its build asked for, the assignment pending on it (novox/hq
+		// issue 325). unassign declares no build, so a call giving it is refused before this.
+		if verb == "assign" && on("build") {
+			argv = append(argv, "--build")
+		}
+		return argv, nil
 	case "pin":
 		if err := need("node", "provision", "from", "module"); err != nil {
 			return nil, err