[App] Moved app.py to root dir for `lightning init app <app_name>` template

[adam-lightning]

What does this PR do?

Currently when You init a new app using lightning init app <app_name> the app.py is in the module directory which impacts running app with --cloud flag (as the requirements.txt is in different dir. Here, we move app.py up to achieve the following templated structure:

tree my-app-test
my-app-test
├── LICENSE
├── README.md
├── app.py                     # <--- now here - previously was created inside my_app_test directory 
├── my_app_test
│   ├── __init__.py
│   └── components
│       ├── component_a
│       │   ├── __init__.py
│       │   └── component_a.py
│       └── component_b
│           ├── __init__.py
│           └── component_a.py
├── requirements.txt
├── setup.py
└── tests
    ├── README.md
    ├── __init__.py
    ├── requirements.txt
    └── test_my_app_test_app.py

Fixes issues with running with --cloud flag. And standardises the folder structure.

Does your PR introduce any breaking changes? If yes, please list them.

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cc @borda

[awaelchli]

Adding an entry in the changelog would be nice if you can :)