Update "app package" to write nested stack template

[efekarakus]

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We want to modify the app package --output-dir command to also write the nested stack generated from the addons/ directory.

For example:

$ ecs-preview app package --name webhook --env test --output-dir infrastructure/
$ ls infrastructure/
webhook-stack.yml         # Already exists
params-test.json          # Already exists
addons-template.yml       # New!

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