Remove files_dir from public API

[danielhfrank]

files_dir existed before main_dir, and has slightly different semantics, but over time we have learned from usage, that we basically do not use files_dir, and thus to avoid confusion we should remove it from the public API.

It's easy for users to get turned around about how to access artifacts, we should remove the files_dir attribute from the list of suggestions that their editor will present them :)