fix(turbopack): ignore underscore_started path (#48792)

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### Why?

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### What?

This PR changes turbopack's appDir source lookup to honor existing
behavior to ignore `_` started path. With this changes, turbopack able
to pass test
`test/e2e/app-dir/underscore-ignore-app-paths/underscore-ignore-app-paths.test.ts`,
fixes WEB-844.

Internally it simply amends two parts, first if given path starts with
_, do not add those into entrypoints to it will not be served. If path
starts with `%5F` instead, replacing path to _ so it can be served.

---------

Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>

packages/next-swc/crates/next-core/src/app_structure.rs

@@ -378,8 +378,11 @@ async fn get_directory_tree(
                 }
             }
             DirectoryEntry::Directory(dir) => {
-                let result = get_directory_tree(dir, page_extensions);
-                subdirectories.insert(basename.to_string(), result);
+                // appDir ignores paths starting with an underscore
+                if !basename.starts_with('_') {
+                    let result = get_directory_tree(dir, page_extensions);
+                    subdirectories.insert(get_underscore_normalized_path(basename), result);
+                }
             }
             // TODO(WEB-952) handle symlinks in app dir
             _ => {}
@@ -694,6 +697,12 @@ async fn directory_tree_to_entrypoints_internal(
     Ok(EntrypointsVc::cell(result))
 }
 
+/// ref: https://github.com/vercel/next.js/blob/c390c1662bc79e12cf7c037dcb382ef5ead6e492/packages/next/src/build/entries.ts#L119
+/// if path contains %5F, replace it with _.
+fn get_underscore_normalized_path(path: &str) -> String {
+    path.replace("%5F", "_")
+}
+
 /// Returns the global metadata for an app directory.
 #[turbo_tasks::function]
 pub async fn get_global_metadata(