Judge an ask by the builder's say the walk read, never a second read (issue 1886, review round 1)

A second read that failed, or that turned true in between, refused the ask and
failed the whole plan.

cmd/mesh-controller/plan_keys.go

@@ -51,6 +51,20 @@ type planKeyReads interface {
 	SaysReferences(ctx context.Context) (bool, error)
 }
 
+// builderSayKey carries, on an ask's context, the builder's say the walk read before asking (withBuilderSay).
+type builderSayKey struct{}
+
+// withBuilderSay hands the say a walk read to the ask's guard, so the guard never reads it a second time.
+func withBuilderSay(ctx context.Context, says bool) context.Context {
+	return context.WithValue(ctx, builderSayKey{}, says)
+}
+
+// builderSayOf is the say a walk handed the ask, and whether it handed one.
+func builderSayOf(ctx context.Context) (says, handed bool) {
+	says, handed = ctx.Value(builderSayKey{}).(bool)
+	return says, handed
+}
+
 // builderSaysReferences is whether a running builder instance says, in its own bucket of instances, that the keys it
 // writes name references (issue 1886). Until one does, a walk asks build-agent for every module, as before R4e: the
 // keys the running builder writes could never be delivered from. A variable so a test needs no bus.