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

might also work with relative packages and other local package aliases.
My point was more that I hadn't looked to see how it was called, so I wasn't sure if it might be used in some situation where `default-package-name` didn't mean "current package name". After a bit more thought, it does seem to be the right package to use though, since it is used to look up unqualified symbols it seems reasonable to look for local packages there too.

I don't remember exactly what binding `*package*` needed to affect, but it looks like just `parse-completion-arguments` would be enough. In that case it could be limited to just the first call to `guess-package`, since I suppose there could be some pathological case where it would affect the case where it looks up `default-package-name`. Looks like sbcl complains about packages containing local nicknames that shadow that package but allows it with a restart, not sure if something like that could happen with allegro relative packages or not.

Not sure if passing it as an argument to `guess-package` is better or not, and if so whether it should be passed as a package object or just the name.