Add method to get packages involved in a `NoSolutionError` (#14457)
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## Summary
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In pixi we overlay the PyPI packages over the conda packages and we
sometimes need to figure out what PyPI packages are involved in the
no-solution error. We could parse the error message, but this is pretty
error-prone, so it would be good to get access to more information. A
lot of information in this module is private and should probably stay
this way, but package names are easy enough to expose. This would help
us a lot!
I collect into a HashSet to remove duplication, and did not want to
expose a rustc_hash datastructure directly, thats's why I've chosen to
expose as an iterator :)
Let me know if any changes need to be done, and thanks!
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Co-authored-by: Zanie Blue <contact@zanie.dev>
