allow link to work with unpublished packages

[edeustace]

when setting up a pie package that depends on an external package that hasn't been published yet, the developer should be able to yarn link in the dependency, then run pie serve|info in the pie package without having to publish the dependency.

Once the dependency has been published, link works. This is only if the dependency has yet to be published.

Workaround: use a file path as the dependency for now, the remove it once the package has been published.