fix(turbopack): don't match empty route groups (#57647)

### What?

Previously the matching just used the last match for children which could lead to empty groups or groups without page matches overriding the actual page.

This PR makes sure this doesn't happen and emits an issue (which unfortunately doesn't get displayed yet) in case there are 2 `page` matches which would go into the children slot.

Closes WEB-1895

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

@@ -400,6 +400,25 @@ pub struct LoaderTree {
     pub global_metadata: Vc<GlobalMetadata>,
 }
 
+#[turbo_tasks::value_impl]
+impl LoaderTree {
+    /// Returns true if there's a page match in this loader tree.
+    #[turbo_tasks::function]
+    pub async fn has_page(&self) -> Result<Vc<bool>> {
+        if self.segment == "__PAGE__" {
+            return Ok(Vc::cell(true));
+        }
+
+        for (_, tree) in &self.parallel_routes {
+            if *tree.has_page().await? {
+                return Ok(Vc::cell(true));
+            }
+        }
+
+        Ok(Vc::cell(false))
+    }
+}
+
 #[derive(
     Clone, PartialEq, Eq, Serialize, Deserialize, TraceRawVcs, ValueDebugFormat, Debug, TaskInput,
 )]
@@ -425,6 +444,10 @@ fn is_parallel_route(name: &str) -> bool {
     name.starts_with('@')
 }
 
+fn is_group_route(name: &str) -> bool {
+    name.starts_with('(') && name.ends_with(')')
+}
+
 fn match_parallel_route(name: &str) -> Option<&str> {
     name.strip_prefix('@')
 }
@@ -677,14 +700,10 @@ async fn directory_tree_to_loader_tree(
         tree.segment = "children".to_string();
     }
 
-    let mut has_page = false;
-
     if let Some(page) = (app_path == for_app_path)
         .then_some(components.page)
         .flatten()
     {
-        has_page = true;
-
         // When resolving metadata with corresponding module
         // (https://github.com/vercel/next.js/blob/aa1ee5995cdd92cc9a2236ce4b6aa2b67c9d32b2/packages/next/src/lib/metadata/resolve-metadata.ts#L340)
         // layout takes precedence over page (https://github.com/vercel/next.js/blob/aa1ee5995cdd92cc9a2236ce4b6aa2b67c9d32b2/packages/next/src/server/lib/app-dir-module.ts#L22)
@@ -751,9 +770,26 @@ async fn directory_tree_to_loader_tree(
                 continue;
             }
 
-            // TODO: detect duplicate page in group segment
-            if !has_page {
+            // skip groups which don't have a page match.
+            if is_group_route(subdir_name) && !*subtree.has_page().await? {
+                continue;
+            }
+
+            if !tree.parallel_routes.contains_key("children") {
                 tree.parallel_routes.insert("children".to_string(), subtree);
+            } else {
+                // TODO: improve error message to have the full paths
+                DirectoryTreeIssue {
+                    app_dir,
+                    message: Vc::cell(format!(
+                        "You cannot have two parallel pages that resolve to the same path. Route \
+                         {} has multiple matches in {}",
+                        for_app_path, app_page
+                    )),
+                    severity: IssueSeverity::Error.cell(),
+                }
+                .cell()
+                .emit();
             }
         } else if let Some(key) = parallel_route_key {
             bail!(
@@ -772,7 +808,7 @@ async fn directory_tree_to_loader_tree(
                 ..Default::default()
             }
             .cell();
-        } else if components.layout.is_some() || current_level_is_parallel_route {
+        } else if current_level_is_parallel_route {
             // default fallback component
             tree.components = Components {
                 default: Some(