Add method to get packages involved in a `NoSolutionError` (#14457)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

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!

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-resolver/src/error.rs

@@ -3,6 +3,7 @@ use std::fmt::Formatter;
 use std::sync::Arc;
 
 use indexmap::IndexSet;
+use itertools::Itertools;
 use owo_colors::OwoColorize;
 use pubgrub::{
     DefaultStringReporter, DerivationTree, Derived, External, Range, Ranges, Reporter, Term,
@@ -409,6 +410,15 @@ impl NoSolutionError {
         }
         Ok(())
     }
+
+    /// Get the packages that are involved in this error.
+    pub fn packages(&self) -> impl Iterator<Item = &PackageName> {
+        self.error
+            .packages()
+            .into_iter()
+            .filter_map(|p| p.name())
+            .unique()
+    }
 }
 
 impl std::fmt::Debug for NoSolutionError {