This PR Fixes#4920
So the problem is that when a next.js application is built on windows, the `pages-manifest.json` file is created with backslashes. If this built application is deployed to a linux hosting enviroment, the server will fail when trying to load the modules.
```
Error: Cannot find module '/user_code/next/server/bundles\pages\index.js
```
My simple solution is to modify the `pages-manifest.json` to always use linux separator (`/`), then also
modify `server/require.js` to, when requiring page, replace any separator (`\` or `/`) with current platform-specific file separator (`require('path').sep`).
The fix in `server/require.js` would be sufficient, but my opinion is that having some cross-platform consistency is nice.
This change was tested by bulding an application in windows and running it in linux and windows, aswell as building an application in linux and running it in linux and windows. The related tests was also run.
# Conflicts:
# test/integration/production/test/index.test.js
* Remove flow-typed
* Remove flow types
* Remove the last types
* Bring back taskr dependency
* Revert "Bring back taskr dependency"
This reverts commit 38cb95d7274d63fe63c6ac3c95ca358a28c17895.
* Bring back preset-flow as it’s used for tests
* Revert "Revert "Bring back taskr dependency""
This reverts commit b4c933ef133f4039f544fb10bf31d5c95d3b27a2.
