Implement PackageService + tests

[2martens]

This task is about implementing the PackageService class. Please take a look at the UML diagram in the meta repository.

This task contains a list of TODOs, each representing a logical sub exercise that can be tackled one at a time. They stand in the chronological order in which they should be worked on. In parentheses you can find the estimated net work amount required. This is a rough estimate and can be quite a bit off (in either direction). When you work on the issue, please update the text with the actual net work time you needed. This can then be used for improvements in upcoming estimations.

• define public API of class (estimated 1.5 to 2 hours amount of work)
• write test class (estimated 2 hours amount of work)
• implement class including test iterations (estimated 2 to 3 hours amount of work)
• add to service.yml (estimated 0.5 hours amount of work)
• write documentation in docs repository (estimated 1.5 to 2 hours amount of work)