Provide a way to copy files in exportPathMap (#5089)

Related #4659 

Adds the possibility for users to copy files inside of `exportPathMap`. This allows for adding `robots.txt` `sitemap.xml` etc. another use case is for https://github.com/hanford/next-offline, currently it's manually reading the buildId in `exportPathMap`.

To allow users to do this we'll introduce a new parameter holding an object with the following keys:

- `dev` - `true` when `exportPathMap` is being called in development. `false` when running `next export`. In development `exportPathMap` is used to define routes and behavior like copying files is not required.
- `dir` - Absolute path to the project directory
- `outDir` - Absolute path to the `out` directory (configurable with `-o` or `--outdir`). When `dev` is `true` the value of `outDir` will be `null`.
- `distDir` - Absolute path to the `.next` directory (configurable using the `distDir` config key)
- `buildId` - The buildId the export is running for

Example usage:

```js
// next.config.js
const fs = require('fs')
const {join} = require('path')
const {promisify} = require('util')
const copyFile = promisify(fs.copyFile)

module.exports = {
  exportPathMap: async function (defaultPathMap, {dev, dir, outDir, distDir, buildId}) {
    if(dev) {
      return defaultPathMap
    }
    // This will copy robots.txt from your project root into the out directory
    await copyFile(join(dir, 'robots.txt'), join(outDir, 'robots.txt'))
    return defaultPathMap
  }
}
```

README.md

@@ -1479,7 +1479,7 @@ next build
 next export
 ```
 
-By default `next export` doesn't require any configuration. It will generate a default `exportPathMap` containing the routes to pages inside the `pages` directory. 
+By default `next export` doesn't require any configuration. It will generate a default `exportPathMap` containing the routes to pages inside the `pages` directory. This default mapping is available as `defaultPathMap` in the example below. 
 
 If your application has dynamic routes you can add a dynamic `exportPathMap` in `next.config.js`.
 This function is asynchronous and gets the default `exportPathMap` as a parameter.
@@ -1538,6 +1538,36 @@ For an example, simply visit the `out` directory and run following command to de
 now
 ```
 
+### Copying custom files
+
+In case you have to copy custom files like a robots.txt or generate a sitemap.xml you can do this inside of `exportPathMap`.
+`exportPathMap` gets a few contextual parameter to aid you with creating/copying files: 
+
+- `dev` - `true` when `exportPathMap` is being called in development. `false` when running `next export`. In development `exportPathMap` is used to define routes and behavior like copying files is not required.
+- `dir` - Absolute path to the project directory
+- `outDir` - Absolute path to the `out` directory (configurable with `-o` or `--outdir`). When `dev` is `true` the value of `outDir` will be `null`.
+- `distDir` - Absolute path to the `.next` directory (configurable using the `distDir` config key)
+- `buildId` - The buildId the export is running for
+
+```js
+// next.config.js
+const fs = require('fs')
+const {join} = require('path')
+const {promisify} = require('util')
+const copyFile = promisify(fs.copyFile)
+
+module.exports = {
+  exportPathMap: async function (defaultPathMap, {dev, dir, outDir, distDir, buildId}) {
+    if (dev) {
+      return defaultPathMap
+    }
+    // This will copy robots.txt from your project root into the out directory
+    await copyFile(join(dir, 'robots.txt'), join(outDir, 'robots.txt'))
+    return defaultPathMap
+  }
+}
+```
+
 ### Limitation
 
 With `next export`, we build a HTML version of your app. At export time we will run `getInitialProps` of your pages.

export/index.js

@@ -10,6 +10,11 @@ import { setAssetPrefix } from '../lib/asset'
 import * as envConfig from '../lib/runtime-config'
 
 export default async function (dir, options, configuration) {
+  function log (message) {
+    if (options.silent) return
+    console.log(message)
+  }
+
   dir = resolve(dir)
   const nextConfig = configuration || loadConfig(PHASE_EXPORT, dir)
   const distDir = join(dir, nextConfig.distDir)
@@ -64,17 +69,6 @@ export default async function (dir, options, configuration) {
     )
   }
 
-  // Copy dynamic import chunks
-  if (existsSync(join(distDir, 'chunks'))) {
-    log('  copying dynamic import chunks')
-
-    await mkdirp(join(outDir, '_next', 'webpack'))
-    await cp(
-      join(distDir, 'chunks'),
-      join(outDir, '_next', 'webpack', 'chunks')
-    )
-  }
-
   // Get the exportPathMap from the config file
   if (typeof nextConfig.exportPathMap !== 'function') {
     console.log(`> No "exportPathMap" found in "${CONFIG_FILE}". Generating map from "./pages"`)
@@ -114,7 +108,7 @@ export default async function (dir, options, configuration) {
     nextExport: true
   }
 
-  const exportPathMap = await nextConfig.exportPathMap(defaultPathMap)
+  const exportPathMap = await nextConfig.exportPathMap(defaultPathMap, {dev: false, dir, outDir, distDir, buildId})
   const exportPaths = Object.keys(exportPathMap)
 
   for (const path of exportPaths) {
@@ -146,9 +140,4 @@ export default async function (dir, options, configuration) {
 
   // Add an empty line to the console for the better readability.
   log('')
-
-  function log (message) {
-    if (options.silent) return
-    console.log(message)
-  }
 }

server/index.js

@@ -155,7 +155,7 @@ export default class Server {
       // So that the user doesn't have to define a custom server reading the exportPathMap
       if (this.dev && this.nextConfig.exportPathMap) {
         console.log('Defining routes from exportPathMap')
-        const exportPathMap = await this.nextConfig.exportPathMap({}) // In development we can't give a default path mapping
+        const exportPathMap = await this.nextConfig.exportPathMap({}, {dev: true, dir: this.dir, outDir: null, distDir: this.distDir, buildId: this.buildId}) // In development we can't give a default path mapping
         for (const path in exportPathMap) {
           const {page, query = {}} = exportPathMap[path]
           routes[path] = async (req, res, params, parsedUrl) => {