[turbopack] Reduce the size of module graph datastructures (vercel#79706)

## Refactor module graph to reduce memory usage

### What?
This PR refactors the module graph implementation to reduce memory usage by:
1. Changing `graph_idx` from `usize` to `u32` to reduce memory footprint
2. Removing unused fields from `SingleModuleGraphModuleNode` (layer and issues)
3. Moving layer resolution to the point of use in dynamic imports and server actions

This is a small progression in runtime:

![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/AwJ29EfoPcPdLSwCZxAz/d669023b-9ac4-44d5-ac9f-54e42b71d341.png)

### Why?
These changes optimize memory usage in the module graph, which is particularly important for large applications. By removing unused fields and using more compact types, we can reduce the memory footprint of the graph.

Notably, SingleModuleGraphNode is now 24 bytes instead of  48.  To make it smaller still we would need to reduce the size of visited moduel, perhaps by storing a reference to the graph that did the original visit?  This would be tricky from a lifetimes perspective but much more optimal.

Author: lukesandberg

crates/next-api/src/dynamic_imports.rs

@@ -125,12 +125,12 @@ pub async fn map_next_dynamic(graph: Vc<SingleModuleGraph>) -> Result<Vc<Dynamic
         .await?
         .iter_nodes()
         .map(|node| async move {
-            let SingleModuleGraphModuleNode { module, layer, .. } = node;
-
-            if layer
-                .as_ref()
-                .is_some_and(|layer| &**layer == "app-client" || &**layer == "client")
-            {
+            let SingleModuleGraphModuleNode { module } = node;
+            let layer = match module.ident().await?.layer {
+                Some(l) => Some(l.await?),
+                None => None,
+            };
+            if layer.is_some_and(|layer| *layer == "app-client" || *layer == "client") {
                 if let Some(dynamic_entry_module) =
                     ResolvedVc::try_downcast_type::<NextDynamicEntryModule>(*module)
                 {

crates/next-api/src/server_actions.rs

@@ -435,14 +435,17 @@ pub async fn map_server_actions(graph: Vc<SingleModuleGraph>) -> Result<Vc<AllMo
         .iter_nodes()
         .map(|node| {
             async move {
-                let SingleModuleGraphModuleNode { module, layer, .. } = node;
-
+                let SingleModuleGraphModuleNode { module } = node;
+                let layer = match module.ident().await?.layer {
+                    Some(l) => Some(l.await?),
+                    None => None,
+                };
                 // TODO: compare module contexts instead?
-                let layer = match &layer {
-                    Some(layer) if &**layer == "app-rsc" || &**layer == "app-edge-rsc" => {
+                let layer = match layer {
+                    Some(layer) if *layer == "app-rsc" || *layer == "app-edge-rsc" => {
                         ActionLayer::Rsc
                     }
-                    Some(layer) if &**layer == "app-client" => ActionLayer::ActionBrowser,
+                    Some(layer) if *layer == "app-client" => ActionLayer::ActionBrowser,
                     // TODO really ignore SSR?
                     _ => return Ok(None),
                 };