Mark baseline that project is metadataless

From @jecisc :
This allows Metacello to better know the current version of the github project and not reload it when the right version is already in the image.
On a project with a lot of dependencies, it tries to load Mocketry around 40 times because this support is not there I think. Because I have multiple projects depending on Mocketry and for each project transitively depending on it, it tries to load it...