Refactor cargo wrapper

Cleans up, splits up and modernize the code:

• Typing
• More constants, less magic (and repeated) values
• Replace shelling out with "native" methods whenever possible, i.e. except running cargo itself
• Tuples when possible
• Single quotes

Some choices might feel "opinionated". I'll gladly debate.

Also makes the registry path argument optional for prepare-debian, its default value is SYSTEM_REGISTRY.