Related to this, I keep bumping into this design problem as I upgrade my codebase to 0.7.
I have an ORM (SearchLight) which supports multiple backends (MySQL, SQLite, etc). I would like to avoid having all the backends as package dependencies (otherwise the user will have to install all of them, despite needing just one). But asking the user to do a pkg> add MySQL or adding MySQL to her project is not enough, is it? It seems that a user of SearchLight would have to add MySQL as a dependency of SearchLight as well?!
Also, it seems that doing a pkg> add will overwrite the Manifest.toml for packages which are being checked-out with pkg> develop and changes in dependencies are lost.