Move all implementation details to _internal

[jack-burridge-cfh]

Currently most packages have the implementation in the main package, would it not be better to put all packages in _internal like metrics? This would reduce the number of public symbols when adding new features and give a simpler route to removing public symbols when we move to 2.x

@srikanthccv @lzchen

[srikanthccv]

We are doing this for the logs as well. We can follow this for any new additions to the codebase. Do you have any reliable suggestions for the existing packages that are already declared stable without introducing any breaking changes?

[jack-burridge-cfh]

Nothing, in particular, it's just less likely to make errors, a recent error would be the removal of the transitive dependency dataclasses.
This thought is all going off all the stuff I had to do for the combined Protobuf encoding.
Moving to have all packages have _internal would make the eventual move to 2.x more straightforward, all the deprecations could be removed more manageably, and you could do it incrementally ready for a 2.x release candidate.

[jack-burridge-cfh]

I think we could make everything _internal in a stable fashion using noqa where necessary.

[lzchen]

Might be something to revisit when we do decide to update to 2.x

[jack-burridge-cfh]

Worth raising issues as lower priority?