rename lib/maintainers-list.nix into maintainers/maintainer-list.nix

Many commits unrelated to `lib` touch that file, this will make `git log ./lib` much saner.

This is what I meant in https://github.com/NixOS/nixpkgs/pull/36119#issuecomment-370184101.

lib/default.nix

@@ -21,7 +21,7 @@ let
 
     # packaging
     customisation = callLibs ./customisation.nix;
-    maintainers = import ./maintainers-list.nix;
+    maintainers = import ../maintainers/maintainer-list.nix;
     meta = callLibs ./meta.nix;
     sources = callLibs ./sources.nix;
     versions = callLibs ./versions.nix;