Function: define-package

Define a new package.
NAME-STRING is the name of the package, as a string.
VERSION-STRING is the version of the package, as a string.
DOCSTRING is a short description of the package, a string.
REQUIREMENTS is a list of dependencies on other packages.
Each requirement is of the form (OTHER-PACKAGE OTHER-VERSION),
where OTHER-VERSION is a string.

EXTRA-PROPERTIES is currently unused.