Bug: `sam build`and `sam package` commands are not building resources recursively for stacksets

### Description:

Unsure if this is a bug report or a feature request, but I have successfully deployed nested applications with `sam` I would expect the build a package steps for stacksets to look similar, so I think it is a bug.

The `sam build` and `sam package` steps for templates containing `AWS::CloudFormation::StackSet` resources are not generating the necessary artifacts for deploying the stackset resource with `sam` and I think this is because they are not looking into the templates recursively for these steps.

### Steps to reproduce:

Filestructure:
```bash
.
+-- src
| +-- traffic_forwarder_pkg
| +-- traffic_forwarder.py
| +-- traffic_forwarder_lib
| +-- lib_file.py
| +-- requirements.txt
+-- template.yaml
+-- traffic_forwarder_template.yaml
```

I have an `AWS::CloudFormation::StackSet` resource in `template.yaml` that looks similar to the following:
```yaml
TrafficForwarderStackSet:
Type: AWS::CloudFormation::StackSet
Properties:
StackSetName: !Sub ${AWS::StackName}-TrafficForwarderStackSet
PermissionModel: SELF_MANAGED
ManagedExecution:
Active: true
Capabilities:
- CAPABILITY_AUTO_EXPAND
- CAPABILITY_IAM
TemplateURL: ./traffic_forwarder_template.yaml
StackInstancesGroup:
- DeploymentTargets:
Accounts: [target_accounts]
Regions:
- !Ref AWS::Region
```
The local template for the stackset contains a serverless function that uses a local file path and looks like the following:

```yaml
TrafficForwarder:
Type: AWS::Serverless::Function
Properties:
Runtime: python3.8
CodeUri: ./src/traffic_forwarder_pkg
Handler: traffic_forwarder.lambda_handler
```

### Observed result:

When `sam build` is run, the following build directory is produced in the `.aws-sam` directory:
```bash
build
+-- lambda_build_1/
+-- lambda_build_2/
+-- lambda_build_3/
+-- template.yaml
```
With no `traffic_forwarder_template.yaml` file or `TrafficForwarder` directory to be found.

This problem cascades into the package and deploy step since there are no resources to be found in the build directory.

### Expected result:

I would expect Lambda deployment packages for the lambda located in the stackset template to be in the build directory after build is run. Then package should upload those packages to S3 and replace code uri in all appropriate locations with the S3 URI.

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)
```json
# Paste the output of `sam --info` here
{
"version": "1.71.0",
"system": {
"python": "3.8.13",
"os": "macOS-13.1-arm64-arm-64bit"
},
"additional_dependencies": {
"docker_engine": "Not available",
"aws_cdk": "Not available",
"terraform": "1.3.7"
}
}
```