Provide source code location (e.g. URL) for detected packages

This has been reported as a desired feature in scancode.io https://github.com/nexB/scancode.io/issues/87 :
- when a package is detected, I would like to know what is the URL to download the source code
- this would presumably then be used to fetch and scan the corresponding sources.

scancode-toolkit makes a best attempt to infer and craft a download URL for a package and a Package URL for the sources. It does not provide a download URL for the sources. This would be a great addition to support this feature (or this is may be instead a lookup/match things in the future PackageDB)