Move internals into a separate package

Following:

* https://github.com/haskell/pvp/pull/46
* https://nikita-volkov.github.io/internal-convention-is-a-mistake/

The move could be considered a breaking change.

It's also not entirely clear what the process/impact is, since we're dealing with a boot package.

@Bodigrim @bgamari @mpickering