[feature] Sandboxed concurrent modules

[roberth]

Multi-module projects can be of problematic size.
Building the modules incrementally, correctly and in parallel would be very helpful, but isn't always considered feasible because of fear that the concurrency breaks the build in subtle ways.
What if mvn2nix could support this? By making the individual modules hermetic, this becomes feasible.
Due to the restrictions of module hermeticity, it may require some extra configuration by the project authors, but unlike with normal concurrent builds, the task is to solve a deterministic issue with inspectable intermediate results, which is far easier.
As a bonus, the build can even be distributed.