Make mode should prioritise modules with many reverse depedencies

Motivation

Suppose we have a module graph that we are compiling with -j2, as in the diagram above. Assume that each module takes the same amount of time to compile. We want to make sure that B and C don't get compiled before A, since then when A gets compiled 1 of 2 cores will be left with no work to do (as D and E can't be started until A finishes).

As far as I can tell, GHC often runs into situations like this. It cannot efficiently use the parallelism it is granted, since it doesn't prioritise work that enables future work.

Proposal

A relatively easy way to improve the situation would be for workers to always compile the available module with the most transitive reverse dependencies.

This is a greedy strategy that tries to maximise the amount of modules available to compile, hoping that this backlog can weather us through bottlenecks in the module graph.