Only respect preferences across the same indexes (#9302)

## Summary

The issue here is fairly complex. Consider the following:

```toml
[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.12.0"
dependencies = []

[project.optional-dependencies]
cpu = [
  "torch>=2.5.1",
  "torchvision>=0.20.1",
]
cu124 = [
  "torch>=2.5.1",
  "torchvision>=0.20.1",
]

[tool.uv]
conflicts = [
  [
    { extra = "cpu" },
    { extra = "cu124" },
  ],
]

[tool.uv.sources]
torch = [
  { index = "pytorch-cpu", extra = "cpu", marker = "platform_system != 'Darwin'" },
]
torchvision = [
  { index = "pytorch-cpu", extra = "cpu", marker = "platform_system != 'Darwin'" },
]

[[tool.uv.index]]
name = "pytorch-cpu"
url = "https://download.pytorch.org/whl/cpu"
explicit = true
```

When solving this project, we first pick a PyTorch version from PyPI, to
solve the `cu124` extra, selecting `2.5.1`.

Later, we try to solve the `cpu` extra. In solving that extra, we look
at the PyTorch CPU index. Ideally, we'd select `2.5.1+cpu`... But
`2.5.1` is already a preference. So we choose that.

Now, we only respect preferences for explicit indexes if they came from
the same index.

Closes https://github.com/astral-sh/uv/issues/9295.

crates/uv-requirements/src/upgrade.rs

@@ -7,7 +7,7 @@ use uv_configuration::Upgrade;
 use uv_fs::CWD;
 use uv_git::ResolvedRepositoryReference;
 use uv_requirements_txt::RequirementsTxt;
-use uv_resolver::{Lock, Preference, PreferenceError};
+use uv_resolver::{Lock, LockError, Preference, PreferenceError};
 
 #[derive(Debug, Default)]
 pub struct LockedRequirements {
@@ -63,7 +63,11 @@ pub async fn read_requirements_txt(
 }
 
 /// Load the preferred requirements from an existing lockfile, applying the upgrade strategy.
-pub fn read_lock_requirements(lock: &Lock, upgrade: &Upgrade) -> LockedRequirements {
+pub fn read_lock_requirements(
+    lock: &Lock,
+    install_path: &Path,
+    upgrade: &Upgrade,
+) -> Result<LockedRequirements, LockError> {
     let mut preferences = Vec::new();
     let mut git = Vec::new();
 
@@ -74,7 +78,7 @@ pub fn read_lock_requirements(lock: &Lock, upgrade: &Upgrade) -> LockedRequireme
         }
 
         // Map each entry in the lockfile to a preference.
-        preferences.push(Preference::from_lock(package));
+        preferences.push(Preference::from_lock(package, install_path)?);
 
         // Map each entry in the lockfile to a Git SHA.
         if let Some(git_ref) = package.as_git_ref() {
@@ -82,5 +86,5 @@ pub fn read_lock_requirements(lock: &Lock, upgrade: &Upgrade) -> LockedRequireme
         }
     }
 
-    LockedRequirements { preferences, git }
+    Ok(LockedRequirements { preferences, git })
 }