That it can be used also for helper modules that are not technically part of the package but are used to instrument the build/test/publish pipeline

This isn’t necessarily true. You can put it further inside with the source code also or in its own non root source root i.e. inside some other python package. If you let pants do its thing, it will be able to figure out what to package as a part of the application code and what not in most of the cases.