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
}
}
```
