Generate javadoc jar for painless's public API (#22704)

The simplest way to do that is to move the public API into a
new package and generate javadoc for that package.

modules/lang-painless/build.gradle

@@ -52,3 +52,15 @@ integTest {
     setting 'script.max_compilations_per_minute', '1000'
   }
 }
+
+// Build
+task apiJavadoc(type: Javadoc) {
+  source = sourceSets.main.allJava
+  include '**/org/elasticsearch/painless/api/'
+  destinationDir = new File(docsDir, 'apiJavadoc')
+}
+task apiJavadocJar(type: Jar) {
+  classifier = 'apiJavadoc'
+  from apiJavadoc
+}
+assemble.dependsOn apiJavadocJar

modules/lang-painless/src/main/java/org/elasticsearch/painless/Definition.java

@@ -21,6 +21,7 @@ package org.elasticsearch.painless;
 
 import org.apache.lucene.util.Constants;
 import org.apache.lucene.util.SetOnce;
+import org.elasticsearch.painless.api.Augmentation;
 
 import java.io.InputStream;
 import java.io.InputStreamReader;

modules/lang-painless/src/main/java/org/elasticsearch/painless/WriterConstants.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.painless;
 
 import org.apache.lucene.search.Scorer;
+import org.elasticsearch.painless.api.Augmentation;
 import org.elasticsearch.search.lookup.LeafDocLookup;
 import org.objectweb.asm.Handle;
 import org.objectweb.asm.Opcodes;

modules/lang-painless/src/main/java/org/elasticsearch/painless/api/Augmentation.java

@@ -17,7 +17,7 @@
  * under the License.
  */
 
-package org.elasticsearch.painless;
+package org.elasticsearch.painless.api;
 
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;

modules/lang-painless/src/main/java/org/elasticsearch/painless/api/package-info.java

@@ -0,0 +1,23 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.
+ */
+
+/**
+ * Java code publicly exposed to Painless code.
+ */
+package org.elasticsearch.painless.api;