Convert values to Starlark values when concating string_list_dict

type.

This is because Dict checks that all the values put in it are Starlark
valid values. But string_list_dict value is of type List<String>, when
Java's List isn't a Starlark value.

Fixes #23065

src/main/java/com/google/devtools/build/lib/packages/Type.java

@@ -620,11 +620,11 @@ public Object copyAndLiftStarlarkValue(
 
     @Override
     public Map<KeyT, ValueT> concat(Iterable<Map<KeyT, ValueT>> iterable) {
-      Dict.Builder<KeyT, ValueT> output = new Dict.Builder<>();
+      ImmutableMap.Builder<KeyT, ValueT> builder = ImmutableMap.builder();
       for (Map<KeyT, ValueT> map : iterable) {
-        output.putAll(map);
+        builder.putAll(map);
       }
-      return output.buildImmutable();
+      return builder.buildKeepingLast();
     }
 
     @Override

src/test/java/com/google/devtools/build/lib/packages/TypeTest.java

@@ -372,6 +372,9 @@ public void testStringListDict() throws Exception {
     assertThat(converted).isEqualTo(expected);
     assertThat(converted).isNotSameInstanceAs(expected);
     assertThat(collectLabels(Types.STRING_LIST_DICT, converted)).isEmpty();
+
+    assertThat(Types.STRING_LIST_DICT.concat(Arrays.asList(converted)))
+      .isEqualTo(expected);
   }
 
   @Test