The Module System does not solve any constraints, nor configure anything. It only combine configurations.
What most people expect from package managers is to choose between conflicting versions, which Nixpkgs avoided so far, or these have been hard-coded using some of the override functions and multiple installs from Nix.
Solving constraints between packages configuration is the source of the complexity I had mentioned previously, behind having a single module system for all of Nixpkgs.