This came up in #413892 and it can DRY with an existing ad hoc definition.

The existing call site was less advanced, lacking the "/foo/" case, but that won't be a problem, because it's arguably correct, and it only occurs in a diagnostic message anyway, and I've tested the message (see commit).
(Also toString and + "/default.nix" commute.)

Tested with:

1. Replace the callPackageWith call by null, to simulate an ancient Nix
2. Run the following commands in a terminal in nixpkgs:

   $ mkdir test/
   
   $ echo '{ asdfasdfasdf }: null' >test/default.nix
   
   $ nix repl -f .
   
   nix-repl> callPackage ./test { }
   error:
         … while calling the 'abort' builtin
           at /home/user/src/nixpkgs/lib/customisation.nix:312:7:
             311|     else
             312|       abort "lib.customisation.callPackageWith: ${error}";
               |       ^
             313|
   
         error: evaluation aborted with the following error message: 'lib.customisation.callPackageWith: Function called without required argument "asdfasdfasdf" at /home/user/src/nixpkgs/test/default.nix'
   

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Nixpkgs 25.11 Release Notes (or backporting 24.11 and 25.05 Nixpkgs Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• NixOS 25.11 Release Notes (or backporting 24.11 and 25.05 NixOS Release notes)
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other contributing documentation in corresponding paths.

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