We had a discussion about this topic some time ago, but I can't find anything now. But there have been similar proposals involving a specially formatted comment or using pragma directives. Another (seamless but more limited) approach was to scan the list of import statements and automatically infer dependencies (+ probably some additional means to do fine tuning). In general I think support for single file projects is a good idea, but I'm not really sure yet which is the best way to go about it.
However, registering single-file packages in the public registry is something that I would likely exclude as a possibility, as the benefit of saving on a single additional file doesn't really justify the additional implementation/maintainance/complexity costs. That is, especially considering that creating the separate git repository for a project that is needed for registration should easily outweigh the effort needed to create a separate package.json file.
Having said that, I need to put some serious thought into this topic to clear my head up, and, because I'm currently still very busy with other things, I just wanted to quickly post my initial thoughts instead of staying silent until that has happened.
TL;DR Worthwhile feature, but the design details should be carefully verified from all possible angles before putting out an implementation.