[connectors-sdk] BaseConnectorSettings support for connectors as module

[Powlinett]

__main__.__file__ is not always available at module import time, but is at runtime. So, my main contribution was to do the tests to confirm that the connectors-sdk can work with the connectors developed as modules.

Proposed changes

• extract the logic to locate config files from _SettingsLoader.settings_customise_sources to their own methods
• raise meaningful error if __main__.__file__ is not available at runtime (for very rare cases)
• update unit tests to really read from filesystem (not mocking files' content anymore)

Related issues

• [connectors-sdk] BaseConnectorSettings does not work with connector as module #5295

Checklist

• I consider the submitted work as finished
• I have signed my commits using GPG key.
• I tested the code for its functionality using different use cases
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

[Kakudou]

Thanks, every things looks good to me, some small QoL suggestions, but nothing really important.

All tests working on my side, 🚀

[Kakudou]

All good for me, ty for the QoL improvements you've added !