I think the main reason why the foo.rs foo/bar.rs stuff was done was so that the tabs in an editor have informative headings. Instead of ten times mod.rs you have foo.rs, baz.rs, test.rs and so on. Personally I don't like it either (for the same reason as you) but the old way is still possible, even on the 2018 edition, so that's what I use.
I think the main reason why the foo.rs foo/bar.rs stuff was done was so that the tabs in an editor have informative headings. Instead of ten times mod.rs you have foo.rs, baz.rs, test.rs and so on. Personally I don't like it either (for the same reason as you) but the old way is still possible, even on the 2018 edition, so that's what I use.