Force overlaps between chunks to be explicit and deterministic

From https://storyboard.baserock.org/#!/story/95:

Way back when, Morph would warn if there were any overlapping files between different chunks. & I believe the initial plan was to make this an error eventually. But since stuff worked anyway, nobody ever took this enforcement further.

We should raise an error if chunks overlap files with each other. However since stuff will break, this should be opt-in, and there should be a way to 'whitelist' certain overlaps. A system could list which version of /bin/awk should be installed in the final system, for example, so that when this happens it's (a) deterministic and (b) explicit