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/main.go

@@ -371,6 +371,15 @@ func (b builds) Built(ctx context.Context, result link.BuildResult) error {
 		return nil
 	}
 	manifest, _, err := takeIn(ctx, b.inv, result)
+	// The assignments pending on this build, made or ended (novox/hq issue 325), said in the daemon's log
+	// after what became of the build.
+	if b.open != nil {
+		defer func() {
+			for _, line := range settlePendingOnBuild(ctx, b.open, result, manifest.Module, err) {
+				fmt.Printf("%s: %s\n", result.ID, line)
+			}
+		}()
+	}
 	// When it was asked, so a plan takes as its outcome only a build asked for it or after it
 	// (novox/hq 04-ISSUES/219). Zero when the id does not say.
 	asked, _ := link.BuildAskedAt(result.ID)