Flatten and separate public/plugin utilities

[maxfischer2781]

The cobald namespace is currently a deeply nested hodgepodge of public/plugin utilities for "users", internal high-level components, and convoluted implementation details. This makes it hard to use, discover, document and maintain. The layout should be refactored to have a consistent structure.

• A "use this" sub-package for external projects, e.g. cobald.plugins and perhaps cobald.protocols
• Per major component a "high level API" sub-package
• Moving all implementations to _private named sub-packages