Add Package URL field to top-level package output

[steven-esser]

Currently, scancode-toolkit returns all the needed details to construct a Package URL when running a scan with the --package option. However, a purl field is not present for top-level packages.

This should be added, as all the necessary fields are present and we return purl values for package dependencies, so we should keep our top-level packages consistent with these dependencies.

[pombredanne]

This has been fixed and merged in develop