If multiple indices contain the same version, use the first index (#5288)

This fixes resolving packages that publish an invalid stub to pypi, such
as tensorrt-llm.

## Summary

In https://github.com/astral-sh/uv/pull/3138 , we implemented
`unsafe-best-match`. However, it seems to not quite work as expected.
When multiple indices contain the same version, it's not clear which
index the current code uses. This PR fixes that to use the first index
the package is in.

## Test Plan

```console
$ echo 'tensorrt-llm==0.11.0' | ./target/debug/uv pip compile - --extra-index-url https://pypi.nvidia.com --python-version=3.10 --index-strategy=unsafe-best-match --annotation-style=line
```

crates/uv-resolver/src/candidate_selector.rs

@@ -232,8 +232,20 @@ impl CandidateSelector {
                 Self::select_candidate(
                     version_maps
                         .iter()
-                        .map(|version_map| version_map.iter().rev())
+                        .enumerate()
-                        .kmerge_by(|(version1, _), (version2, _)| version1 > version2),
+                        .map(|(map_index, version_map)| {
+                            version_map.iter().rev().map(move |item| (map_index, item))
+                        })
+                        .kmerge_by(
+                            |(index1, (version1, _)), (index2, (version2, _))| match version1
+                                .cmp(version2)
+                            {
+                                std::cmp::Ordering::Equal => index1 < index2,
+                                std::cmp::Ordering::Less => false,
+                                std::cmp::Ordering::Greater => true,
+                            },
+                        )
+                        .map(|(_, item)| item),
                     package_name,
                     range,
                     allow_prerelease,
@@ -242,8 +254,20 @@ impl CandidateSelector {
                 Self::select_candidate(
                     version_maps
                         .iter()
-                        .map(VersionMap::iter)
+                        .enumerate()
-                        .kmerge_by(|(version1, _), (version2, _)| version1 < version2),
+                        .map(|(map_index, version_map)| {
+                            version_map.iter().map(move |item| (map_index, item))
+                        })
+                        .kmerge_by(
+                            |(index1, (version1, _)), (index2, (version2, _))| match version1
+                                .cmp(version2)
+                            {
+                                std::cmp::Ordering::Equal => index1 < index2,
+                                std::cmp::Ordering::Less => true,
+                                std::cmp::Ordering::Greater => false,
+                            },
+                        )
+                        .map(|(_, item)| item),
                     package_name,
                     range,
                     allow_prerelease,