Move internals into a separate package

[hasufell]

Following:

• Formalize ".Internal" module convention pvp#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

[Bodigrim]

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

One would have to add a new submodule and patch Hadrian to pick it up, but I do not expect any blockers.

[jdkr]

There's one question that arises to me. What would be considered as Internal? Only the modules named Internal, or the whole tree of modules located under System.OsPath and System.OsString?
As far as I understand, the modules in OsPath are the implementation of the Abstract File Path Proposal (which may change in the future) and the modules in OsString are platform specific. Both I would consider Internal.

[hasufell]

#200