perf: Refactor recursiveReadDirSync (#52517)

- Use `opendirSync` instead of `readdirSync`
- Use `.slice` instead of `.replace` as the rootDir

Before (1.54ms):

![CleanShot 2023-07-10 at 18 33 23@2x](https://github.com/vercel/next.js/assets/3676859/829e0a49-35da-4754-bc3f-6fe243e815f4)

With the change (0.88ms):

![CleanShot 2023-07-10 at 18 47 43@2x](https://github.com/vercel/next.js/assets/3676859/cabe1fdd-5861-49ba-8c8d-b505c16fbf7a)

This scales when public/ folder grows.

packages/next/src/server/lib/recursive-readdir-sync.ts

@@ -1,5 +1,5 @@
 import fs from 'fs'
-import { join } from 'path'
+import { sep } from 'path'
 
 /**
  * Recursively read directory
@@ -10,20 +10,22 @@ export function recursiveReadDirSync(
   dir: string,
   /** This doesn't have to be provided, it's used for the recursion */
   arr: string[] = [],
-  /** Used to replace the initial path, only the relative path is left, it's faster than path.relative. */
-  rootDir = dir
+  /** Used to remove the initial path suffix and leave only the relative, faster than path.relative. */
+  rootDirLength = dir.length
 ): string[] {
-  const result = fs.readdirSync(dir, { withFileTypes: true })
-
-  result.forEach((part) => {
-    const absolutePath = join(dir, part.name)
+  // Use opendirSync for better memory usage
+  const result = fs.opendirSync(dir)
 
+  let part: fs.Dirent | null
+  while ((part = result.readSync())) {
+    const absolutePath = dir + sep + part.name
     if (part.isDirectory()) {
-      recursiveReadDirSync(absolutePath, arr, rootDir)
-      return
+      recursiveReadDirSync(absolutePath, arr, rootDirLength)
+    } else {
+      arr.push(absolutePath.slice(rootDirLength))
     }
-    arr.push(absolutePath.replace(rootDir, ''))
-  })
+  }
 
+  result.closeSync()
   return arr
 }