DJC: Design an enhanced DejaCode Package model to identify source code relationships

The working idea here is to come up with the best way to identify cross-package relationships, especially to be able to get to
(1) the source code
and
(2) more complete copyright+license data, which usually comes from the source code.

We could start by displaying the values for `contains_source_code`, `source_packages`, `code_view_url`, and `vcs_url` in the "Detected Package" section of the Scan tab (when a value is available). The ScanCode package model has this support for source code relationships (which are also in PurlDB):

- the `contains_source_code` boolean flags tells if the package itself contains source code: https://github.com/nexB/scancode-toolkit/blob/0465269543eb338086c10bdeb1e81d3013522b4d/src/packagedcode/models.py#L452
- the `source_packages` field is a list of Package URLs that may exist for this package https://github.com/nexB/scancode-toolkit/blob/0465269543eb338086c10bdeb1e81d3013522b4d/src/packagedcode/models.py#L457
- the `code_view_url` and `vcs_url` provide reference URLs to view or fetch actual source code https://github.com/nexB/scancode-toolkit/blob/0465269543eb338086c10bdeb1e81d3013522b4d/src/packagedcode/models.py#L414

Now that we have standardized on PURL as the package identifier, we should be able to pursue this DejaCode improvement using **package-set** values via integration with the PurlDB.