Motivation
A development of #242.

One of the really nasty things about source layouts in Python is the level of duplcation required to get it right. setuptools has

[tool.setuptools.packages.find]
where = ["src"]

and hatch has src implicitly by default.

And then we have more duplication with tools, for example

[tool.coverage.run]
source = [ "src" ]

[tool.ruff]
src = [ "src" ]

Wouldn't it be great if this was available at the project.layout = ["src"] level with a PEP specification or similar? That's a very complicated question but I think we can at least have a [tool.usethis.layout] and make the problem worse while allowing us to at least keep all these fields in sync and effectively migrate between layouts.

Summary of desired enhancement
Consider the above and how we want to handle layouts in general.