find symbols in local package nicknames in swank-c-p-c

might also work with relative packages and other local package aliases.
Your description of the semantics of `default-package-name` make sense to me. Your second paragraph suggests that perhaps `current-package-name` would be a better name for the argument? (Or did you mean something else?)

About the comment, one thing that is not obvious to me is what exactly is affected by the binding of `*package*`. Is it the `parse-completion-arguments` function? If so, should the binding be there? (Or maybe even further down in `guess-package`?)