. The loop stops when no leaves are left: all
    remaining packages have at least one dependency left in the graph.

    Then we continue with the remaining graph, by taking the length for the
    longest path to any node from root, ignoring any paths that contain a single
    node twice (i.e. cycles). This is done through a depth-first search through
    the graph, while keeping track of the path to the node.

    Cycles in the graph result would result in node being revisited while also
    being on its own path. In this case, take no action. This helps ensure we
    don't get stuck in a cycle.

    When assigning weight, the longer path (i.e. larger length) is preferred.

    We are only interested in the weights of packages that are in the
    requirement_keys.
    