Include global --config when determining namespace packages (#9603)

## Summary

When determining whether _any_ settings have namespace packages, we need
to consider the global settings (as would be provided via `--config`).
This was a subtle fallout of a refactor.

Closes #9579.

## Test Plan

Tested locally by compiling Ruff and running against this
[namespace-test](https://github.com/gokay05/namespace-test) repo.

crates/ruff_workspace/src/resolver.rs

@@ -173,8 +173,7 @@ impl<'a> Resolver<'a> {
         // Determine whether any of the settings require namespace packages. If not, we can save
         // a lookup for every file.
         let has_namespace_packages = self
-            .settings
-            .values()
+            .settings()
             .any(|settings| !settings.linter.namespace_packages.is_empty());
 
         // Search for the package root for each file.