Export all symbols from the main package

[llucax]

What's needed?

This repository started as a container for multiple unrelated utilities, but now it is coming together as part of the BaseApiClient. Imports start to get a bit messy as one needs to pick some stuff from here and there, but all clients will need to import from all those subpackages, so it doesn't make sense to keep them separated.

Proposed solution

Make all subpackages private and export all symbols directly in frequenz.client.base.