code: def add_package_dependency(self, package_name, version):
          
          if not PEP440_VERSION_PATTERN.match(version):
              raise ValueError('Invalid Version: "{}"'.format(version))
          self.dependencies.add(PackageDependency(package_name, version))
docs: Add a package to the list of dependencies.
  
          :param package_name: The name of the package dependency
          :type package_name: str
          :param version: The (minimum) version of the package
          :type version: str