Add DerivationTree.packages() -> HashSet<&P>

[konstin]

In uv, we check the available versions to determine if there a prerelease versions and show an appropriate hint if required, mainly collecting package versions here to display a hint here.

This is the most efficient way of writing this function, but the derivation tree is small in the number of packages involved.

[Eh2406]

This seems like a useful thing to have. It should probably have some tests. Separately it seems similar to code in errors_the_same_with_only_report_dependencies, perhaps this method could be used there.

[konstin]

I added .packages() to the existing error test case to avoid duplicating the setup code, let me know if there's a better place for it.