Allow to enrich project metadata manually (think of project curations)

[sschuberth]

Some package managers provide no project metadata in their definition files besides direct dependencies only. And example for this are Python requirements.txt definition files. It would be nice to have a mechanism, e.g. inside .ort.yml, to enrich such projects with additional metadata, like a name and version. Such a mechanism could also help to remedy some cases of #9417 as it allows for making project IDs unique.

The same mechanism should also allow to attach metadata to unmanaged projects, like declaring a license.