The documentation for pkgs.lib.lists.unique warned me that the function
is O(n^2). So I conducted some rather unscientific tests using my own
Kubernetes flake and found no noticeable time penalty:
`rm result && nix-collect-garbage -d && nix build` always took
between 37 and 40 seconds, with or without gatherNamespaces.
But, well, n=10.
Also, make more obvious the flaw in mk.namespaces.
If I prefix something with "N.B." it shouldn't be hidden in the code.