ort-project-file: Further improvements

[fviernau]

findings / ideas

1. The Vcs.revision property could be made optional: if absent, ORT
   would use the version string vs. tag name matching to determine the revision. (edit: this case works with empty string
   already)
2. The SourceArtifact.hash property could also be made optional, and
   the hash could be computed by ORT on the fly if absent.
3. OrtProject.dependencies could also be made optional, to support projects without dependencies.
4. The documentation in ORT model's project class says, that Project.vcs comes from the metadata.
   However, the implementation derives it from the VCS corresponding to the definitionFilePath. Should
   Project.vcs be left empty?
5. If projectName is not defined, then "unknown" will be used as name.
   This can lead to id collisions / analyzer issues. Better would be to make the id's unique
   by including the path from analyzer root into the id.