Auto merge of #5100 - alexcrichton:better-dep-ordering, r=matklad

Improve Cargo's scheduling of builds

Historically Cargo has been pretty naive about scheduling builds, basically just
greedily scheduling as much work as possible. As pointed out in #5014, however,
this isn't guaranteed to always have the best results. If we've got a very deep
dependency tree that would otherwise fill up our CPUs Cargo should ideally
schedule these dependencies first. That way when we reach higher up in the
dependency tree we should have more work available to fill in the cracks if
there's spare cpus.

Closes #5014