Refactor _check_import_or_attribute

[AlexWaygood]

No functional change -- just a small refactor to reduce repetition and (hopefully) make the function slightly more efficient

[github-actions]

This change has no effect on typeshed. 🤖🎉

[srittau]

I'm not super happy with either the previous version nor this new version. Both have these if/elif/else cascades with the occasional return in between, which make it hard to follow the program flow. I wonder whether extracting functions like _check_typing_attribute() and _check_typing_extensions_attribute() would make the flow easier to understand.

That said, this is already an improvement to the status quo.

[AlexWaygood]

I wonder whether extracting functions like _check_typing_attribute() and _check_typing_extensions_attribute() would make the flow easier to understand.

yeah, I wondered about that too. I'll give it a go as a separate PR, then we can decide which one we like more

[AlexWaygood]

Superseded by #440