Restructure dependency/release data model in dependency checker

Currently we have a `Dependency` class that may/not be github and has various raises etc accordingly.

If it is a github dep we can create a "release" object for it which unlocks further functionality.

I think we need to create a GithubDependency and separate it from Dependency, likewise with Release, so we can make some of the functionality available to other deps (cf #320 )