This makes sure plugins like @zeit/next-typescript or @zeit/next-mdx don't have to provide the `hot-self-accept-loader`.
This fixes for example @zeit/next-mdx, which doesn't implement `pageExtensions` but we do use mdx as top level pages for https://github.com/zeit/docs
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.
