lib/types: make DAG entries mergeable

Removes the `uniq` constraint on `after` and `before` so that we can
merge multiple definitions for the same DAG entry:

    {
      dag = mkMerge [
        {
          foo = lib.hm.dag.entryBefore [ "bar" ] {
            # definition 1
          };
        }
        {
          foo = lib.hm.dag.entryBefore [ "qux" ] {
            # definition 2
          };
        }
        {
          foo = {
            # definition 3
          };
        }
      ];
    }

In this example `foo` will come before `bar` and `qux`.

tests/lib/types/dag-merge-result.txt

@@ -1,3 +1,4 @@
 before:before
+merged:left,middle,middle,right
 between:between
 after:after

tests/lib/types/dag-merge.nix

@@ -1,7 +1,8 @@
 { config, lib, pkgs, ... }:
 
 let
-  inherit (lib) concatStringsSep hm mkIf mkMerge mkOption types;
+  inherit (lib)
+    concatStringsSep hm mkIf mkMerge mkBefore mkAfter mkOption types;
 
   dag = lib.hm.dag;
 
@@ -11,17 +12,22 @@ let
   in concatStringsSep "\n" data + "\n";
 
 in {
-  options.tested.dag = mkOption { type = hm.types.dagOf types.str; };
+  options.tested.dag = mkOption { type = hm.types.dagOf types.commas; };
 
   config = {
     tested.dag = mkMerge [
-      { never = mkIf false "never"; }
+      (mkIf false { never = "never"; })
+      { never2 = mkIf false "never2"; }
       { after = mkMerge [ "after" (mkIf false "neither") ]; }
       { before = dag.entryBefore [ "after" ] (mkIf true "before"); }
       {
         between =
           mkIf true (dag.entryBetween [ "after" ] [ "before" ] "between");
       }
+      { merged = dag.entryBefore [ "between" ] "middle"; }
+      { merged = mkBefore "left"; }
+      { merged = dag.entryBetween [ "after" ] [ "before" ] (mkAfter "right"); }
+      { merged = dag.entryBefore [ "between" ] "middle"; }
     ];
 
     home.file."result.txt".text = result;