The prepares for next-server.
I also took this as an opportunity to get all build directory paths from a single location, as they were previously scattered across webpack/babel plugins and loaders.
Previously we called this directory holding the pages/chunks for server rendering `.next/dist` instead of `.next/server` which is confusing both when looking at it and in the codebase, since there's also `distDir` as a configuration option.
Also made this a constant in `next/constants` so functionality using this can be easily found.
* Add build manifest
* Split out css since they don’t have exact name
* Remove pages map
* Fix locations test
* Re-run tests
* Get consistent open ports
* Fix static tests
* Add comment about Cache-Control header
* Initial implementation of next export without exportPathMap
* Shorter message
* Set up flow
* Create pages manifest
* Use pagesManifest for next export
* Fix tests
* Document defaultPathMap
* Replacing the path is no longer needed
* Use posix normalize for consistent behaviour
* Remove second instance of examples
* Add comment about what pages-manifest does
* Make windows path a route
* Allow next.config.js to export a function
* Expose phases to the configuration function
* Use same value as variable name
* Add next/constants
* Add documentation for config function / phases
* Add constants.js to npm bundle
