I'm in the middle of working out analog and mixed-signal flows and figuring out how to handle dependencies. Ultimately, I think the right answer is that each repository should remain its own project in some top level "projects" directory, and that other projects should either be told where it is or clone it themselves. For now, I would recommend just cloning that project and then referencing it in xschem by adding its xschem directory to the paths in xschemrc.