Simplify the dependent_packages_url field

[joshbressers]

Today if I visit the dependent_packages_url field, I get a pretty comprehensive JSON document (I'm not going to add the URL as it looks like an expensive query). For packages that are depended on many other packages, that URL often times out

I'm looking for a simple list of dependencies or dependents (I can work with either, as I can construct the graph once I have the list). It would be fine to just reference the id integer if that's easier and more light weight