Ref T10010. We currently require withSlugs() to have properly formatted slugs, but this leads to similar code in several places.
Instead: accept any slug, normalize slugs in the query, return a map so callers can figure out what happened if they want.
This tends to do the right thing by default, while keeping enough information around to do more complex things if necessary. A similar approach for querying commits has worked well in Diffusion.