Add relative app dir field (#46393)

This adds a relative app dir field to the required files manifest so
that we don't rely on absolute paths from a cached build since the cache
can be restored in a separate context where the value no longer applies.

x-ref: #45864

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

packages/next/src/build/index.ts

@@ -896,6 +896,9 @@ export default async function build(
           )
         )
 
+      const outputFileTracingRoot =
+        config.experimental.outputFileTracingRoot || dir
+
       const manifestPath = path.join(distDir, SERVER_DIRECTORY, PAGES_MANIFEST)
 
       const requiredServerFiles = nextBuildSpan
@@ -923,6 +926,7 @@ export default async function build(
             },
           },
           appDir: dir,
+          relativeAppDir: path.relative(outputFileTracingRoot, dir),
           files: [
             ROUTES_MANIFEST,
             path.relative(distDir, manifestPath),
@@ -1926,8 +1930,8 @@ export default async function build(
 
             const root =
               config.experimental?.turbotrace?.contextDirectory ??
-              config.experimental?.outputFileTracingRoot ??
-              dir
+              outputFileTracingRoot
+
             const nextServerEntry = require.resolve(
               'next/dist/server/next-server'
             )
@@ -2172,9 +2176,6 @@ export default async function build(
         )
       )
 
-      const outputFileTracingRoot =
-        config.experimental.outputFileTracingRoot || dir
-
       if (config.output === 'standalone') {
         await nextBuildSpan
           .traceChild('copy-traced-files')

test/production/standalone-mode/required-server-files/required-server-files.test.ts

@@ -321,6 +321,8 @@ describe('should set-up next', () => {
     expect(typeof requiredFilesManifest.config.configFile).toBe('undefined')
     expect(typeof requiredFilesManifest.config.trailingSlash).toBe('boolean')
     expect(typeof requiredFilesManifest.appDir).toBe('string')
+    // not in a monorepo so relative app dir is empty string
+    expect(requiredFilesManifest.relativeAppDir).toBe('')
   })
 
   it('should de-dupe HTML/data requests', async () => {