Optimize next-app-loader resolving speed (vercel#50745)

## What?

We recently implemented an optimized resolving method for `app` in Turbopack, this ports some of the main changes in that resolving logic to optimize `next-app-loader` which during compilation resolves the tree structure that we use to render in `app-render.tsx`.

Here's the results for a page that is nested a few levels deep on vercel.com using App Router. These results only cover `next-app-loader`, not any modules compiled below it.

### Before

<img width="671" alt="CleanShot 2023-06-03 at 22 36 26@2x" src="https://github.com/vercel/next.js/assets/6324199/0edeb060-2460-4a7d-95a7-1c22ea26a065">

### After

<img width="673" alt="CleanShot 2023-06-03 at 22 55 10@2x" src="https://github.com/vercel/next.js/assets/6324199/f40964fc-b169-4d95-8711-73cbff3ec76a">


## Raw numbers

<table>
<tr>
 <td>Before</td>
 <td>After</td>
 <td>Delta</td>
 <td>Delta (percent)</td>
</tr>
<tr>
 <td>1.620 ms</td>
 <td>76.39 ms</td>
 <td>-1.543.61 ms</td>
  <td>-95.2%</td>
</tr>
</table>

## How?

Changed the resolving logic to use `fileExists`, looping over the provided pageExtensions.
For Turbopack we have a process that does only one pass for generating all trees. That also only reads directories instead of checking individual files, which is even better (<5ms for generating all possible trees) but this PR is a quick win that has a big impact already without refactoring the entire entries generation in webpack.

packages/next/src/build/index.ts

@@ -28,7 +28,7 @@ import {
   PAGES_DIR_ALIAS,
   INSTRUMENTATION_HOOK_FILENAME,
 } from '../lib/constants'
-import { fileExists } from '../lib/file-exists'
+import { FileType, fileExists } from '../lib/file-exists'
 import { findPagesDir } from '../lib/find-pages-dir'
 import loadCustomRoutes, {
   CustomRoutes,
@@ -588,7 +588,7 @@ export default async function build(
           for (const page in mappedPages) {
             const hasPublicPageFile = await fileExists(
               path.join(publicDir, page === '/' ? '/index' : page),
-              'file'
+              FileType.File
             )
             if (hasPublicPageFile) {
               conflictingPublicFiles.push(page)

packages/next/src/build/webpack/loaders/metadata/discover.ts

@@ -1,4 +1,3 @@
-import type webpack from 'webpack'
 import type {
   CollectingMetadata,
   PossibleStaticMetadataFileNameConvention,
@@ -7,6 +6,7 @@ import path from 'path'
 import { stringify } from 'querystring'
 import { STATIC_METADATA_IMAGES } from '../../../../lib/metadata/is-metadata-route'
 import { WEBPACK_RESOURCE_QUERIES } from '../../../../lib/constants'
+import { MetadataResolver } from '../next-app-loader'
 
 const METADATA_TYPE = 'metadata'
 
@@ -16,16 +16,14 @@ async function enumMetadataFiles(
   filename: string,
   extensions: readonly string[],
   {
-    resolvePath,
-    loaderContext,
+    metadataResolver,
     // When set to true, possible filename without extension could: icon, icon0, ..., icon9
     numericSuffix,
   }: {
-    resolvePath: (pathname: string) => Promise<string>
-    loaderContext: webpack.LoaderContext<any>
+    metadataResolver: MetadataResolver
     numericSuffix: boolean
   }
-) {
+): Promise<string[]> {
   const collectedFiles: string[] = []
 
   const possibleFileNames = [filename].concat(
@@ -36,19 +34,9 @@ async function enumMetadataFiles(
       : []
   )
   for (const name of possibleFileNames) {
-    for (const ext of extensions) {
-      const pathname = path.join(dir, `${name}.${ext}`)
-      try {
-        const resolved = await resolvePath(pathname)
-        loaderContext.addDependency(resolved)
-
-        collectedFiles.push(resolved)
-      } catch (err: any) {
-        if (!err.message.includes("Can't resolve")) {
-          throw err
-        }
-        loaderContext.addMissingDependency(pathname)
-      }
+    const resolved = await metadataResolver(path.join(dir, name), extensions)
+    if (resolved) {
+      collectedFiles.push(resolved)
     }
   }
 
@@ -59,16 +47,14 @@ export async function createStaticMetadataFromRoute(
   resolvedDir: string,
   {
     segment,
-    resolvePath,
+    metadataResolver,
     isRootLayoutOrRootPage,
-    loaderContext,
     pageExtensions,
     basePath,
   }: {
     segment: string
-    resolvePath: (pathname: string) => Promise<string>
+    metadataResolver: MetadataResolver
     isRootLayoutOrRootPage: boolean
-    loaderContext: webpack.LoaderContext<any>
     pageExtensions: string[]
     basePath: string
   }
@@ -82,11 +68,6 @@ export async function createStaticMetadataFromRoute(
     manifest: undefined,
   }
 
-  const opts = {
-    resolvePath,
-    loaderContext,
-  }
-
   async function collectIconModuleIfExists(
     type: PossibleStaticMetadataFileNameConvention
   ) {
@@ -96,7 +77,7 @@ export async function createStaticMetadataFromRoute(
         resolvedDir,
         'manifest',
         staticManifestExtension.concat(pageExtensions),
-        { ...opts, numericSuffix: false }
+        { metadataResolver, numericSuffix: false }
       )
       if (manifestFile.length > 0) {
         hasStaticMetadataFiles = true
@@ -116,7 +97,7 @@ export async function createStaticMetadataFromRoute(
         ...STATIC_METADATA_IMAGES[type].extensions,
         ...(type === 'favicon' ? [] : pageExtensions),
       ],
-      { ...opts, numericSuffix: true }
+      { metadataResolver, numericSuffix: true }
     )
     resolvedMetadataFiles
       .sort((a, b) => a.localeCompare(b))