Enhance PrefetchCache with directory init-file resolution

- Added a new field `dir_init` to `PrefetchCache` to store pre-resolved init-file information for directories, allowing for efficient retrieval during prefetch operations.
- Introduced the `prefetch_dir_init` method in the `Vfs` struct to look up init-file info from the prefetch cache.
- Updated the `prefetch_project_files` function to populate the `dir_init` map based on the defined `INIT_FILE_PRIORITY`.
- Adjusted related tests to ensure proper initialization of the `dir_init` field in various scenarios.

These changes improve the performance of directory handling in the VFS by reducing the need for repeated metadata probes.

tests/tests/build.rs

@@ -241,19 +241,34 @@ fn parallel_snapshot_with_prefetch_cache() {
 
     use std::collections::HashMap;
     let mut files = HashMap::new();
+    let mut is_file = HashMap::new();
+    let mut children_map: HashMap<std::path::PathBuf, Vec<std::path::PathBuf>> = HashMap::new();
     for entry in walkdir::WalkDir::new(root).follow_links(true) {
         let entry = entry.unwrap();
         let path = entry.path().to_path_buf();
+        is_file.insert(path.clone(), entry.file_type().is_file());
         if entry.file_type().is_file() {
             files.insert(path.clone(), fs::read(&path).unwrap());
         }
+        if entry.depth() > 0 {
+            if let Some(parent) = entry.path().parent() {
+                children_map
+                    .entry(parent.to_path_buf())
+                    .or_default()
+                    .push(path);
+            }
+        }
+    }
+    for children in children_map.values_mut() {
+        children.sort_by(|a, b| a.file_name().cmp(&b.file_name()));
     }
 
     let vfs_cached = memofs::Vfs::new_default();
     vfs_cached.set_prefetch_cache(memofs::PrefetchCache {
         files,
-        is_file: std::collections::HashMap::new(),
-        children: std::collections::HashMap::new(),
+        is_file,
+        children: children_map,
+        dir_init: std::collections::HashMap::new(),
     });
 
     let snap_cached = librojo::snapshot_from_vfs(&ctx, &vfs_cached, &project_path)
@@ -359,6 +374,7 @@ fn overlapping_path_roots_no_duplicate_children() {
         files,
         is_file,
         children: children_map,
+        dir_init: std::collections::HashMap::new(),
     });
 
     let snap = librojo::snapshot_from_vfs(&ctx, &vfs, &project_path)