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

might also work with relative packages and other local package aliases.
Not sure whether it needs to be mentioned in the docstring or not. I'm not really sure what situations the default-package-name is intended to be used. If it just represents the code being edited, I would say it doesn't need mentioned, since the intent is that it find symbols the same way the implementation would when READing the file: if a package is specified, it is looked up in the context of the current package on implementations where that matters.

Possibly (guess-package default-package-name) isn't the right way to get the 'current package' in that case though?

As far as a comment, that might be a good idea. It might not be obvious why it needs to be bound for people that don't use that sort of feature.