Loader - interface that implements access to plugins, the specific low-level component. The splitter does not know anything about how it works. setup_import_alias() - logically grouped with the Loader, as they both provide tools for working with abstraction - versioned modules directory. If Loader moved to splitter.py, this knowledge will be in two places. 
When we do splitter optional component, the loader will still be - it is not tied to the splitter logically.