Don't apply irrelevant constraints when validating site-packages (#5231)

## Summary

The current code was checking every constraint against every
requirement, regardless of whether they were applicable. In general,
this isn't a big deal, because this method is only used as a fast-path
to skip resolution -- so we just had way more false-negatives than we
should've when constraints were applied. But it's clearly wrong :)

## Test Plan

- `uv venv`
- `uv pip install flask`
- `uv pip install --verbose flask -c constraints.txt` (with `numpy<1.0`)

Prior to this change, Flask was reported as not satisfied.

crates/uv-installer/src/site_packages.rs

@@ -266,6 +266,18 @@ impl SitePackages {
         requirements: &[UnresolvedRequirementSpecification],
         constraints: &[Requirement],
     ) -> Result<SatisfiesResult> {
+        // Collect the constraints.
+        let constraints: FxHashMap<&PackageName, Vec<&Requirement>> =
+            constraints
+                .iter()
+                .fold(FxHashMap::default(), |mut constraints, requirement| {
+                    constraints
+                        .entry(&requirement.name)
+                        .or_default()
+                        .push(requirement);
+                    constraints
+                });
+
         let mut stack = Vec::with_capacity(requirements.len());
         let mut seen = FxHashSet::with_capacity_and_hasher(requirements.len(), FxBuildHasher);
 
@@ -306,8 +318,9 @@ impl SitePackages {
                         }
                         RequirementSatisfaction::Satisfied => {}
                     }
+
                     // Validate that the installed version satisfies the constraints.
-                    for constraint in constraints {
+                    for constraint in constraints.get(&distribution.name()).into_iter().flatten() {
                         match RequirementSatisfaction::check(distribution, &constraint.source)? {
                             RequirementSatisfaction::Mismatch
                             | RequirementSatisfaction::OutOfDate