LUCENE-6766: put Placeholder back so javadocs are OK; deprecate Lucene60Codec

lucene/backward-codecs/src/java/org/apache/lucene/codecs/Placeholder.java

@@ -0,0 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.codecs;
+
+/** Remove this file when adding back compat codecs */
+public class Placeholder {
+  
+}

lucene/backward-codecs/src/java/org/apache/lucene/codecs/lucene60/Lucene60Codec.java

@@ -50,7 +50,9 @@ import org.apache.lucene.codecs.perfield.PerFieldPostingsFormat;
  * @see org.apache.lucene.codecs.lucene60 package documentation for file format details.
  *
  * @lucene.experimental
+ * @deprecated Only for 6.0 back compat
  */
+@Deprecated
 public class Lucene60Codec extends Codec {
   private final TermVectorsFormat vectorsFormat = new Lucene50TermVectorsFormat();
   private final FieldInfosFormat fieldInfosFormat = new Lucene60FieldInfosFormat();