uv-pypi-types: make room for group names in addition to extras

This adds support for providing conflicting group names in addition to
extra names to `Conflicts`.

This merely makes "room" for it in the types while keeping everything
working. We'll add proper support for it in the next commit.

Note that one interesting trick we do here is depend directly on
`hashbrown` so that we can make use of its `Equivalent` trait. This in
turn lets us use things like `ConflictItemRef` as a lookup key for a
hashset that contains `ConflictItem`. This mirrors using a `&str` as a
lookup key for a hashset that contains `String`, but works for arbitrary
types. `std` doesn't support this, but `hashbrown` does. This trick in
turn lets us simplify some of our data structures.

This also rejiggers some of the serde-interaction with the conflicting
types. We now use a wire type to represent our conflicting items for
more flexibility. i.e., Support `extra` XOR `group` fields.

crates/uv-resolver/src/lib.rs

@@ -25,6 +25,13 @@ pub use resolver::{
 pub use version_map::VersionMap;
 pub use yanks::AllowedYanks;
 
+/// A custom `HashSet` using `hashbrown`.
+///
+/// We use `hashbrown` instead of `std` to get access to its `Equivalent`
+/// trait. This lets use store things like `ConflictItem`, but refer to it via
+/// `ConflictItemRef`. i.e., We can avoid allocs on lookups.
+type FxHashbrownSet<T> = hashbrown::HashSet<T, rustc_hash::FxBuildHasher>;
+
 mod bare;
 mod candidate_selector;