Design minimal package and deps output

[pombredanne]

I would like to have a minimalist data structure to support:

• Optionally only collect the PURL of packages, source packages and dependencies in ScanCode Toolkit #3464
  Ideally:
• this should map naturally to a list of purls and scancode's current data structure for package dependencies, including "vers" ranges
• this should be easy to transform (to and from) GH-specific format for https://docs.github.com/en/rest/dependency-graph/dependency-submission