Refactor `DistFinder` to allow handling errors (#709)
For the install tests, i need the ability to ignore failures in the
`DistFinder`. To avoid just copy&pasting a version that collects errors
separately, i followed
https://gendignoux.com/blog/2021/04/01/rust-async-streams-futures-part1.html
and switched the custom channel over to an async stream yielding
`Result` items.
I like the async streams mirror the normal iterator api.
