'Return a list of URLs for different versions of this
package, sorted by version.  A version\'s URL only appears
in this list if it has an explicitly defined URL.'

@memoized
def version_urls(self):

    version_urls = {}
    for v in sorted(self.versions):
        args = self.versions[v]
        if ('url' in args):
            version_urls[v] = args['url']
    return version_urls
