pkg_files and pkg_tar interface are different

We've been using pkg_tar for things we should have been using pkg_files for. In the process of converting, I've been noticing that the 2 have the same concepts, but a different syntax. It would be great to be able to change the rule name and for everything (that is applicable) to just work. Things like `package_dir` -> `prefix`, `remap_paths` -> `renames`.