_loader: Use only one Dependency() class

This moves the Dependency() implementation from the _loader.types module into the _loader.loadelement module, replacing the duplicate Dependency() definition there.

Instead of creating _loader.loadelement.Dependency objects on the fly during the load process, we simply fill in the new LoadElement member on the original Dependency objects.

This refactor should make the code easier to work with, since any originally loaded state from the YAML is preserved in the intermediate LoadElement data model and does not need to be manually handed over to a separate object.