code: def conflicting_deps(tree):
      
      conflicting = defaultdict(list)
      for p, rs in tree.items():
          for req in rs:
              if req.is_conflicting():
                  conflicting[p].append(req)
      return conflicting
docs: Returns dependencies which are not present or conflict with the
      requirements of other packages.
  
      e.g. will warn if pkg1 requires pkg2==2.0 and pkg2==1.0 is installed
  
      :param tree: the requirements tree (dict)
      :returns: dict of DistPackage -> list of unsatisfied/unknown ReqPackage
      :rtype: dict