Better handling of hatch package location declaration

[nathanjmcdougall]

Motivation
Following from #347 and #383; we are explicitly pointing hatch to the package location. So we need some way of deciding layout structure. It's time to make a heuristic function for this, and test the case where the src dir exists but is empty (since in that case hatch will still need explicit help beyond its heuristics).

This needs to be considered as a part of #316. We also don't know how the uv build backend will affect this.

Summary of desired enhancement
Test and improve handling of this case.