Client refactor: directories and naming

[sechkova]

Move the packages under the client directory in a way that makes it easy to distinguish repository components from internal client components and client public API.

Proper naming is needed for the client refactor directory which is planned to land in the main branch in parallel with the current client.

[jku]

Background for these decisions is that we want the new client to co-exist with old one (tuf.client.updater.Updater) so need to think of clever naming.

There are three decisions that have not been made:

• Module (& directory) name. Currently it's client_rework which is not great. Something short and recognizably client-related would be good. "client-ng" is boring but fits the requirements: other options are very welcome.
• Public class (& file) name. Currently it's Updater which is the same class name as the old/current Updater... I think I'm fine with keeping it that but the downside is that the API is different and there's some possibility of confusion.
• How to separate public/private API:
  • In any case I think we want to expose the public classes from __init__.py in client dir: this might be enough (only what's in the module itself is public, any submodules are not?)
  • Additionally we may want to put files that are clearly non-public into a _internal/ directory under client directory

So a model proposal based on those:

tuf/
    client-ng/
        __init__.py
        updater.py
        fetcherinterface.py
        _internal/
            requestsfetcher.py
            download.py
            metadatabundle.py

This is likely to not be final (in that I think there's probably functionality still in updater.py that could be separated out) but would be close enough for merging to develop

[jku]

While we do this we should update tox config, and make it so the directory is omitted from coverage for now
python -m coverage report -m --fail-under 97 --omit "{toxinidir}/tuf/client_rework/*

[sechkova]

#1401 implements a structure based on this proposal but also slightly different in terms of internal classes.

[sechkova]

Closed with #1401.