Fixes#4686
Adds tests for @zeit/next-typescript so that we don't regress on this again.
I've fixed an issue in the `next` CLI too which caused lingering processes when the process gets force killed, which is what we do in the test suite, so it kept running if there was no manual quit.
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.
