FLUID-6737: Using `fluid.rootResolveSingle` as a subcomponent does not result in a single instance of the grade.

Description

I have put an example up on CodePen, which is:

fluid.defaults("examples.rootComponent1", {
    gradeNames: ["fluid.modelComponent", "fluid.resolveRootSingle"],
    singleRootType: "examples.rootComponent1",
    model: {
      value: "root component defaults"
    }
});


fluid.defaults("examples.rootHolder", {
    gradeNames: ["fluid.modelComponent"],
    components: {
      rootComponent: {
        type: "examples.rootComponent1"
      }
    },
    model: {
      value: "{rootComponent}.model.value"
    }
});

var that1 = examples.rootHolder();

console.log("Component model value before change: " + that1.model.value);

that1.applier.change("value", "change applied by first parent");

console.log("Root model value following relayable change: " + that1.model.value);

var that2 = examples.rootHolder();

console.log("Root model value of first component following creation of second parent: " + that1.model.value);
console.log("Root model value of second component following creation of second parent: " + that2.model.value);


that2.applier.change("value", "change applied by first parent");

console.log("Root model value of first component following change by second component: " + that1.model.value);
console.log("Root model value of second component following change by second component: " + that2.model.value);


that1.rootComponent.applier.change("value", "change applied from root subcomponent");

console.log("Root model value of first component following change by root component: " + that1.model.value);
console.log("Root model value of second component following change by root component: " + that2.model.value);

that1.destroy();
that2.destroy();

This outputs:

"Root model value of second component following creation of second parent: root component defaults"

"Root model value of first component following change by second component: change applied by first parent"

"Root model value of second component following change by second component: change applied by first parent"

"Root model value of first component following change by root component: change applied from root subcomponent"

"Root model value of second component following change by root component: change applied by first parent"

From these examples it seems like each subcomponent that uses `fluid.resolveRootSingle` becomes in essence like an ordinary `fluid.modelComponent`. The second one instantiated does not replace the first, and each only relays to and from its parent in its own component tree.

As we've discussed in the chats, I want to have a `fluid.resolveRootSingle` grade as a subcomponent to ensure that the first component that needs the grade causes it to come into existence.