Nested namespace packages support (#10541)

## Summary
PEP 420 says [nested namespace
packages](https://peps.python.org/pep-0420/#nested-namespace-packages)
are allowed, i.e. marking a directory as a namespace package marks all
subdirectories in the subtree as namespace packages.

`is_package` is modified to use `Path::starts_with` and the order of
checks is reversed to do in-memory checks first before hitting the disk.

## Test Plan
Added unit tests. Previously all tests were run with `namespace_packages
== &[]`. Verified that one of the tests was failing before changing the
implementation.

## Future Improvements
The `is_package_with_cache` can probably be rewritten to avoid repeated
calls to `Path::starts_with`, by caching all directories up to the
`namespace_root`:
```ruff
let namespace_root = namespace_packages
    .iter()
    .filter(|namespace_package| path.starts_with(namespace_package))
    .min();
```

crates/ruff_workspace/src/options.rs

@@ -293,8 +293,8 @@ pub struct Options {
     pub builtins: Option<Vec<String>>,
 
     /// Mark the specified directories as namespace packages. For the purpose of
-    /// module resolution, Ruff will treat those directories as if they
-    /// contained an `__init__.py` file.
+    /// module resolution, Ruff will treat those directories and all their subdirectories
+    /// as if they contained an `__init__.py` file.
     #[option(
         default = r#"[]"#,
         value_type = "list[str]",

crates/ruff_workspace/src/resolver.rs

@@ -203,7 +203,7 @@ impl<'a> Resolver<'a> {
 /// A wrapper around `detect_package_root` to cache filesystem lookups.
 fn detect_package_root_with_cache<'a>(
     path: &'a Path,
-    namespace_packages: &'a [PathBuf],
+    namespace_packages: &[PathBuf],
     package_cache: &mut FxHashMap<&'a Path, bool>,
 ) -> Option<&'a Path> {
     let mut current = None;
@@ -219,7 +219,7 @@ fn detect_package_root_with_cache<'a>(
 /// A wrapper around `is_package` to cache filesystem lookups.
 fn is_package_with_cache<'a>(
     path: &'a Path,
-    namespace_packages: &'a [PathBuf],
+    namespace_packages: &[PathBuf],
     package_cache: &mut FxHashMap<&'a Path, bool>,
 ) -> bool {
     *package_cache