Create a heuristic to find project directory structure

[nathanjmcdougall]

Motivation
Toward #386 and related to #316. We need a way to infer project directory structure.

Summary of desired enhancement
We need to document the approach as much as implement it. But ideal is just a function with no args using the cwd to determine the layout, returning a Literal["flat", "src"] or possibly also "unknown" as an option for fallback behaviour.

[nathanjmcdougall]

This actually already exists.