Improve get_interface_attr speed through caching

[fahhem]

Inspired by #1184 but only a small step.

To me, it looks like the car/ module does a lot of (potentially unnecessary) work at import time, such as defining dataclasses for documentation of a car. As a result, collecting the tests has to not only import class and function definitions, but also do real work.

In this case, interfaces.get_interface_attr() walks a folder structure ignoring some folders, dynamically imports modules, does some optional dict-merging, and returns. During test collection, it's called 3 times, each time walking the directory structure and importing the same module.

It seems to me that actually closing #1184 would entail a dozen or more of PRs like this: finding a small improvement to the import-time work or moving it either to runtime or maybe even "compilation".

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