Rename 'makeLoader' -> 'makeRetriever' to distinguish from bus loading
Make 'core' and tests use 'makeRetriever', not 'makeLoader'
Break out 'retriever' into separate file
Keep 'util' as its own testable module, add test for 'findProjRoot' failure
Use default 'retriever' module in 'core' and tests
Refresh 'projRoot' on 'retriever' object after 'chdir'