Support NestedStack inside StackSetStack

Like the title says, if you specify a stack inside a StackSetStack, then the Stack doesn't get deployed. (or it can, using the `--all` flag but then it only gets deployed to the primary region as its own stack and not all target regions).

Here is an example of code that works for deploying a bucket to all regions:

```
const app = new App();
const stack = new Stack(app);

const stackSetStack = new StackSetStack(stack, "StackSetStack");
new Bucket(stackSetStack, "Bucket");

new StackSet(stack, 'StackSet', {
target: StackSetTarget.fromAccounts({
regions: ['eu-west-1', 'eu-west-2'],
accounts: [process.env.CDK_DEFAULT_ACCOUNT!],
}),
template: StackSetTemplate.fromStackSetStack(stackSetStack),
});
```

As expected this deploys all the resources (a bucket in this case) to all the regions.

However, if you add a stack between the bucket and the StackSetStack, it no longer deploys the bucket to the all regions, e.g.

```
const app = new App();
const stack = new Stack(app);

const stackSetStack = new StackSetStack(stack, "StackSetStack");
const subStack = new Stack(stackSetStack, "SubStack");
new Bucket(subStack, "Bucket");

new StackSet(stack, 'StackSet', {
target: StackSetTarget.fromAccounts({
regions: ['eu-west-1', 'eu-west-2'],
accounts: [process.env.CDK_DEFAULT_ACCOUNT!],
}),
template: StackSetTemplate.fromStackSetStack(stackSetStack),
});
```

My expected behaviour would be that this deploys the stack to all regions and all regions subsequently deploy a bucket.

Please correct me if I'm wrong.

Start by reproducing the nested-stack example using StackSetStack, Stack, Bucket, and StackSetTemplate.fromStackSetStack. Trace how the template is assembled when a Stack is nested inside StackSetStack, then verify that the nested stack and its bucket are deployed to every configured target region.

Written by the indexing model from the issue text.