Application Location Object must reference a AWS Serverless Application Repository application.

### Describe your idea/feature/enhancement
I would like to publish a application to the Serverless Application Repository that is using nested stacks without having to create a different Application for each Nested Stack.

My template looks as follows:
```
Resources:
MyNestedApp:
Type: AWS::Serverless::Application
Properties:
Location: s3://cf-templates-utgydnaf2za8-us-east-1/5451e544fdf49c365ee2ca3510611caf.template
```

When running: "sam publish", I get error:
Error: Resource with id [MyNestedApp] is invalid. Location property must be an Application Location Object referencing a valid AWS Serverless Application Repository application.

If I publish the nested template as it's own application in the Serverless Application Repository, as shown below, everything works fine.
```
Resources:
MyNestedApp:
Type: AWS::Serverless::Application
Properties:
ApplicationId: arn:aws:serverlessrepo:us-east-1:{{aws_accountid}}:applications/{{NESTED_TEMPLATE}}
```

I wish SAM CLI would support publishing applications that supported nested stacks without having to publish each Nested Stack individually.