This is useful for cases where the project is made available through other means when e.g. deployed. Mark said:
"Yeah, I needed something like this as well and I think the answer in theory is to declare the dependencies as "provided". Unfortunately, this isn't implemented for inter-project dependencies, but if you open a bug, I'll try to get it implemented for the next version."
See the following for the full thread:
IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#53 for the full issue and to comment.