When you say library system, do you mean something more or less like a separate search path and tools for managing it?

I've written a little about how we can more or less accomplish something like meaningfully-reusable shell libraries in the nix ecosystem. I think https://www.t-ravis.com/post/shell/the_missing_comprehensive... lays out the general idea but you can also pick through how my bashrc integrates libraries/modules (https://github.com/abathur/bashrc.nix).

(I'm just dropping these in bin, since that works with existing search path for source. Not ideal, but the generally explicit nature of dependencies in nix minimizes how much things can leak where they aren't meant to go.)