Link Protocol
The link: protocol lets you connect your project to an external directory.
Links vs portals
Since the target referenced by link: may not exist until postinstall scripts have run, and since Yarn guarantees a predictable behavior regardless of the execution order, we need to disambiguate packages (which contain package.json files, and may list dependencies of their own) from arbitrary folders (which may not).
In practice, this means that link: dependencies can only point to arbitrary folders - in other words, whatever it references cannot have a package.json file, and thus can't have dependencies. If you need to symlink to a package, use portals instead.