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.

def get_dists_to_ignore(installed):
    """
    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.
    """
    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))