Fix flight manifest to include all chunks (#40365)

When a client module is imported by both a module from an `app` entry and a module from `pages` entry, it is possible that it is included in the `pages` chunk eventually. The current check makes the manifest incomplete in that case.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

packages/next/build/webpack/plugins/flight-manifest-plugin.ts

@@ -121,12 +121,6 @@ export class FlightManifestPlugin {
         id: ModuleId,
         mod: webpack.NormalModule
       ) {
-        // if appDir is enabled we shouldn't process chunks from
-        // the pages dir
-        if (chunk.name?.startsWith('pages/') && appDir) {
-          return
-        }
-
         const isCSSModule =
           mod.type === 'css/mini-extract' ||
           (mod.loaders &&