This was a bug with the source build in 4.0, and only manifests when packaging a content-only project.
The version of the editor that we ship through the launcher is compiled using the UE_ROCKET define, which adds -Rocket to the editor’s command line. We use this parameter to control behaviors that are common to people who aren’t building the editor themselves, and accidentally lumped support for packaging of content-only projects into this category.
We’ll have it fixed in the next release.