This is my personal opinion. It should not be taken as a statement from the crates.io team, nor does it imply that anybody else on the team shares my opinion.
I think that's the significantly more common case. Often it's both. diesel-migrations happens to be maintained by the Diesel project, but the more important property is "it's an implementation of database migrations for Diesel". Try searching for chrono- or tokio- or diesel- in the crates.io search. All the top results are things that integrate with or add new behaviors to those projects, and are not maintained by the owner of the main project. (Note: I am not trying to present this as evidence that this is the most common case. This obviously has sampling bias. However I think those are some really good examples of the kinds of crates I'm referring to)
I would like to re-iterate that these are my personal opinions, and I am not speaking on behalf of the team.