So if you have a project called "splitter", your source code really lives under `splitter/splitter/`. I would agree that seems a bit redundant and `splitter/src` look better, but the source code is not in the project root.
I got src directories in the Elixir dependencies written in Erlang, in about 12% of the node_modules used by a React project and in the few C extensions I'm using for Ruby.
May I conclude that src is uncommon at least in scripting languages? (Elixir is compiled.) Maybe the reason is that there is only source code and there is no need for a separate directory for a build / dist.
https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-str...
It describes some of the outdated motivations for the other layouts commonly seen with python, as well as the many benefits of the “src” layout.