Enhance support for Package dependencies

[tdruez]

Current implementation: https://github.com/nexB/scancode.io/blob/main/scanpipe/pipes/scancode.py#L400-L417

The dependencies are stored in the database along the discovered packages without any way to make a distinction from a regular package. This is problematic in many ways and causing more issues than benefits when dealing with the results.

2 steps improvements:

1. Store the dependencies on a new field on the DiscoveredPackage model
2. Implement a proper DependentPackage model related to the DiscoveredPackage

https://github.com/nexB/scancode-toolkit/blob/develop/src/packagedcode/models.py#L386

https://github.com/nexB/scancode-toolkit/blob/3f7da81d6b207ac2b1d384defb83a5f2c82216f4/src/packagedcode/models.py#L308

[tdruez]

See #447 for implementation.