code: def update_extend(dst, src):
      
      for k, v in src.items():
          existing = dst.setdefault(k, [])
          for x in v:
              if x not in existing:
                  existing.append(x)
docs: Update the `dst` with the `src`, extending values where lists.
  
      Primiarily useful for integrating results from `get_library_config`.