There needs to be a way to tell a Depends item to which sub-project the required product belongs

When using sub-projects to aggregate existing projects, we cannot assume that the product names are globally unique. Changing conflicting product names manually is hardly an option, while auto-uniquifying the names seems even worse.
A sensible solution is to give Depends items an optional property that could be called "project" and which specifies the direct parent project of the respective product. The default would be the current project. Since project names are not necessarily unique either, the project would be specified like a path (absolute or relative to the current project).