Versioning policy for Internal modules

It's been a common practice in the Haskell community to put internal functions to `Internal` modules, which are not meant for a public API but for a semi-private API that can be useful to access library internals to extend the API. I think it's natural to relax the versioning policy for changes in those `Internal` modules. Should we mention it somehow in the PVP?

Start by reading the current PVP specification and the existing comment on this issue. Determine how changes to Internal modules are currently covered and what policy wording would need to change. Done means the policy decision is resolved and the relevant specification text clearly documents the treatment of Internal modules.

Written by the indexing model from the issue text.