Fix deletion of SWANK packages in swank-loader.lisp

(Reported by Ruben Vaz.)

* swank-loader.lisp (list-swank-packages): Catch more SWANK
packages.
(string-starts-with): Refactored out from LIST-SWANK-PACKAGES.
(delete-packages): New function capable of
deleting interdependent packages.
(init): Use it.