hof/gen: better duplicate detection and error messages

[verdverm]

While working on: afd5124

I noticed that the messaging when duplicates are found is inconsistent. We could add the logic in this commit to more places, and more similar logic for finding where duplicates come from.

I don't think this currently works across generators, output files really ought to be recorded at both the generator and runtime level, then we can query and detect cross-generator output duplication, maybe even find a way to merge them, especially for data files where the values can be merged at the CUE level before rendering it once for all generators (i.e. we'd lift this out into the runtime or a special generator for data file collision and merging?

---

• ensure tests for the various conflict cases exist and pass