Fixes#4691Fixes#4614
This PR gives path to https://github.com/zeit/next-plugins/pull/242
I did not add or remove `^` near dependency versions in package.json files. However, I don't exclude that some changes can be made given that rc is more stable than beta.
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.
