Fix build only server-side /api/index.js (#8167)

* Build error for index API routes

* Fix server build

packages/next/build/entries.ts

@@ -68,7 +68,7 @@ export function createEntrypoints(
   Object.keys(pages).forEach(page => {
     const absolutePagePath = pages[page]
     const bundleFile = page === '/' ? '/index.js' : `${page}.js`
-    const isApiRoute = bundleFile.match(API_ROUTE)
+    const isApiRoute = page.match(API_ROUTE)
 
     const bundlePath = join('static', buildId, 'pages', bundleFile)
 

test/integration/api-support/pages/api/index.js

@@ -1,3 +1,6 @@
+/* eslint-disable-next-line */
+import fs from 'fs'
+
 export default (req, res) => {
   res.send('Index should work')
 }