source as instance of dedicated class

[dafyddstephenson]

Currently source (or source_repo in some cases) is a str containing a URL (or soon, a local path). Meanwhile every class with a source-like attribute has a get() method. As we support more flexibility (e.g. local files, roms-tools integration), source should be an instance of a dedicated class, rather than str. This class should have logic for handling remote vs. local, different file types, repos, etc., and could likely also encapsulate a single get() method rather than this being defined separately for multiple different classes.