sexp.lisp: Option to omit a specific package prefix

The symbol package prefix can get really verbose at times, it'd be nice to have an option to omit it.

For instance

```lisp
(export '*local*)
(defvar *local-package* nil
"If `*local-package*' designates a package instance, don't print the package
prefix of its symbols.")

(defun print-symbol (symbol stream)
(let ((package (symbol-package symbol))
(name (prin1-to-string symbol)))
(cond ((eq package +cl-package+) (write-string "CL:" stream))
((eq package +keyword-package+) (write-char #\: stream))
((eq package *local-package*)
;; Don't print the package prefix.
nil)
(package (s-xml:print-string-xml (package-name package) stream)
(write-string "::" stream))
(t (write-string "#:" stream)))
(if (char= (char name (1- (length name))) #\|)
(write-string name stream :start (position #\| name))
(write-string name stream :start (1+ (or (position #\: name :from-end t) -1))))))
```