Git storage extra files

[zangell44]

Summary

This PR adds support and documentation for loading external files using Git storage.

Changes

• When extracting a flow from a file, pass the file path as __file__
• Document Git storage use case where additional files are loaded

Importance

Closes #4754

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[chicago-joe]

Will this work with AWS CodeCommit storage as well?

[trymzet]

This will break on a Jupyter notebook with a NameError because Jupyter does not populate __file__.

[zangell44]

@chicago-joe this will not work with CodeCommit, only Git storage at the moment. Happy to chat a bit further about CodeCommit enhancements or workarounds if you'd like to open an issue

[zangell44]

@trymzet you're correct that Jupyter notebooks (and other execution enironments) do not populate __file__. However, I'm not sure I understand the Prefect specific use case you're describing.

We set __file__ when the Flow is loaded from storage to allow users to access __file__ for loading additional files https://github.com/PrefectHQ/prefect/pull/4767/files#diff-8b9309d6d1ca7b8dd179ef8df5c1e02aa0011a639f2d1753f8eee09ca08cc133R87.

For any code executed outside of loading the flow from storage (which should happen when an agent picks up the flow run), __file__ is not used