the use case is that if I have several versions of the private dependency.

I.e I develop a project A, and after a while I find, that part of it wold be useful to break out to another package, so I make a package X and list it as dependencie. Here, 'add-source' works. BUT, some time later I'm done with A, and start developing B, and include X as a dependencie. Then, I find that X could use some improvements -- but after these my project A probably breaks due to these changes. The solution, of course, is versioning, but I think 'add-source' doesn't help there anymore (right?), unless I copy the head to another directory and do the improvements there.

This last mentioned way (of leaving a trail of previous versions) is a manual way of version management. What I was thinking of is that, is there some paved solution available (short of running my own Hackage, which some do, as I've been reading.)

Sorry -- I should have been much more explicit!