Right now we have code that looks like:

    # Where root_dep is a Dependency and deps_loader is a DEPSLoader.
    deps_content = deps_loader.Load(
        root_dep.deps_repo_url,
        root_dep.deps_repo_revision,
        root_dep.deps_file)

That's a code smell since it means common/deps_parser.py is tightly coupled with the guts of common/dependency.py. It'd be a lot cleaner to have a method on the Dependency class which accepts the DEPSLoader as an argument.

Also we have code that closes over root_dep to get at deps_file, but then forces callers to patch things up after the fact with SetParent. Why not just call SetParent directly?