Separate dirs for users and internal ones

[carpawell]

The structure of the repo looks a little bit overwhelming now. We have structs for real users (object, nemap, etc) and some non-SDK things (doc, api, scripts, stat). Have you considered moving "SDK" things to a separate folder? Naming can be hard here but still.

Originally posted by @carpawell in #591 (review)

[carpawell]

It was not a problem before but now we have api, docs, internal, scripts, stat dirs that are not expected to be used by a regular user (pool and waiter are also questionable to me but ok). We can place the all the "user" code to a separate dir or mark non-usable dirs somehow (e.g. start their name with _).

[roman-khimov]

stat is an SDK thing. doc is not an issue. scripts are tolerable. api can either be hidden (internal) or renamed to something more meaningful. But the last thing I'd want is to move any of the packages we have now.

[carpawell]

stat is an SDK thing

How can it be used? It has PoolStat and it allows to create PoolStat, used by the Pool only. Why is it at root level?

scripts are tolerable

Can not (should not) be used by a user that wants to work with neofs and goes to SDK.

But the last thing I'd want is to move any of the packages we have now.

We can move all the "internal" things to internal and leave all the "user" things as is.