Unnest Debusine-specific entries in dput-ng profiles

While working on Debian wiki documentation for our dput-ng integration, I realized that it's unfortunately difficult to override individual items within the nested debusine object in a local profile, because you can only override top-level items that way so the whole debusine object gets replaced. For example, this makes it difficult to override the workflow task data in a local profile.

Since we haven't announced this feature widely yet, I think now is the best time to break compatibility and lay out the profile in a way that's easier to override locally.