code: def get_dists_to_ignore(installed):
      
      installed_keys = {key_from_req(r): r for r in installed}
      return list(flat_map(lambda req: dependency_tree(installed_keys, req), PACKAGES_TO_IGNORE))
docs: Returns a collection of package names to ignore when performing pip-sync,
      based on the currently installed environment.  For example, when pip-tools
      is installed in the local environment, it should be ignored, including all
      of its dependencies (e.g. click).  When pip-tools is not installed
      locally, click should also be installed/uninstalled depending on the given
      requirements.