This commit adds the actual logic for extracting layer groupings and
merging them until the layer budget is satisfied.
The implementation conforms to the design doc as of the time of this
commit.
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).
