Create new model for dependencies

[JonoYang]

scancode-toolkit has a new model called DependentPackage, which models the dependencies detected for a Package from a package manifest. The DependentPackage data shows up in the scancode JSON results in the top-level Dependencies field. We should create a new model, DiscoveredDependency (or some other name) to represent this in scancode.io

[tdruez]

@JonoYang I remember you had committed a new model for the dependencies but we decided to postpone after the toolkit upgrade.
Could you start a new branch with this new model? This would be a good base for discussion.

[JonoYang]

@tdruez I started a new branch with the DiscoveredDependency model I had from earlier. The only change I made from my original commit is that I increased the size of the package_uid related fields to 1024.