As described in the design document, this adds considerations for
closure size and popularity. All closures meeting a certain threshold
for either value will have an extra edge from the image root to
themselves inserted in the graph, which will cause them to be
considered for inclusion in a separate layer.
This is preliminary because popularity is considered as a boolean
toggle (the input I generated only contains the top ~200 most popular
packages), but it should be using either absolute popularity values or
percentiles (needs some experimentation).
