Take no rebuild into a waiting walk, and ask what a tier left unasked (hq issue 754)

A rebuild that landed while a walk waited for mesh-delivery's word was taken as
the walk's build of that module; once let go, the walk asked a tier only when
every module of it was unasked, so it never asked the rest and stood still.

A walk now takes only the outcomes of builds it asked, and none while it waits;
a rebuild does not join a waiting walk and says it runs beside it; and a tier
asked in part is asked the rest, including a module held built that the walk
never asked for, which is somebody else's build and not sent as the walk's.

cmd/mesh-controller/queue_test.go

@@ -637,8 +637,9 @@ func TestAPlanStoppedAtItsFirstMachineIsRetried(t *testing.T) {
 	stopped := inventory.Plan{ID: "plan-rollout", Repository: "novox/a", Branch: "main", Commit: "c0ffee",
 		Created: long, State: inventory.PlanFailed, Tiers: [][]string{{"a"}, {"b"}},
 		Note: "a stopped at its first machine in tier 0: laptop refused what it was sent",
-		Modules: map[string]*inventory.PlanModule{"a": {State: "built", BuiltAt: &long, Commit: "c0ffee",
-			First: []string{"laptop"}, FirstAt: &long, Why: "laptop refused what it was sent"}}}
+		// Asked by the plan, as every module a plan holds built is (novox/hq issue 754).
+		Modules: map[string]*inventory.PlanModule{"a": {State: "built", AskedAt: &long, Build: "build-a", BuiltAt: &long,
+			Commit: "c0ffee", First: []string{"laptop"}, FirstAt: &long, Why: "laptop refused what it was sent"}}}
 	if err := open.inventory.SavePlan(ctx, &stopped); err != nil {
 		t.Fatal(err)
 	}