Rearrange things

What does this merge request do and why?

Rearrange things to open up for future refactoring and improve overall readability.

• Group modules and functions by usecase: promptlib.duo_chat and promptlib.code_suggestions.
• Common functions for both usecases are in promptlib.common module.
• Rearrange tests for follow the new structure.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.