Allow more Flask entrypoints

[javierdelapuente]

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Currently for Flask the app global variables has to be in the file app.py in the base project directory.

This PR allows more possible places for entry points, following what is done in the FastAPI framework.

The app variable for the WSGI entrypoint will be searched in:

1. app.py file in the base directory
2. main.py file in the base directory
3. In any of the following directories, the extension will look for the variable in the files __init__.py, app.py or main.py:
   a. app directory.
   b. src directory.
   c. A directory with the same name as the project.

[linostar]

Can you add integration tests for the following use cases:

• app variable inside one of the allowed entrypoint files other than the default app.py
• same as above but the file that has the app variable is excluded in prime list

[linostar]

Can you also add unit tests for the other use-cases like src/main.py, src/__init__.py, foo_bar/main.py and foo_bar/__init__.py?