lib.types: add a fileset type

lib/types.nix

@@ -66,6 +66,11 @@ let
     fixupOptionType
     mergeOptionDecls
     ;
+  inherit (lib.fileset)
+    isFileset
+    unions
+    empty
+    ;
 
   inAttrPosSuffix =
     v: name:
@@ -612,6 +617,15 @@ let
         };
       };
 
+      fileset = mkOptionType {
+        name = "fileset";
+        description = "fileset";
+        descriptionClass = "noun";
+        check = isFileset;
+        merge = loc: defs: unions (map (x: x.value) defs);
+        emptyValue.value = empty;
+      };
+
       # A package is a top-level store path (/nix/store/hash-name). This includes:
       # - derivations
       # - more generally, attribute sets with an `outPath` or `__toString` attribute