Coalesce parallel rust builds within same project

[nhynes]

for instance, given the following dep graph

a -> b -> c
d

the invocations should be

cargo build --bin c --bin d # this is the desired outcome
cargo build --bin b
cargo build --bin a

but right now c and d are built separately.

Workspace::construct_build_plan would ideally return Vec<Vec<TargetRef>> containing topologically sorted antichains.