Build: Add api jar and client jar for xpack

This adds back (again) building a transport client plugin jar for
x-pack, and also adds producing an "api" jar which extension authors can
build against. For now, both these jars are exactly the same, but
eventually they could differ, and be reduced to less than the real
x-pack jar.

see elastic/stackelastic/elasticsearch#7

Original commit: elastic/x-pack-elasticsearch@0a989de18b

elasticsearch/x-pack/build.gradle

@@ -1,6 +1,9 @@
+import java.nio.charset.StandardCharsets
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.StandardCopyOption
 import org.elasticsearch.gradle.MavenFilteringHack
 import org.elasticsearch.gradle.test.NodeInfo
-import java.nio.charset.StandardCharsets
 
 group 'org.elasticsearch.plugin'
 
@@ -9,6 +12,7 @@ esplugin {
   name 'x-pack'
   description 'Elasticsearch Expanded Pack Plugin'
   classname 'org.elasticsearch.xpack.XPackPlugin'
+  hasClientJar = true
 }
 
 ext.versions = [
@@ -128,6 +132,37 @@ bundlePlugin {
   }
 }
 
+// add api jar for extension authors to compile against
+// note this is just the normal x-pack jar for now, with a different name
+project.afterEvaluate {
+  task apiJar {
+    dependsOn('generatePomFileForApijarPublication', project.jar)
+    doFirst {
+      Path jarFile = project.jar.outputs.files.singleFile.toPath()
+      String apiFileName = jarFile.fileName.toString().replace(project.version, "api-${project.version}")
+      Files.copy(jarFile, jarFile.resolveSibling(apiFileName), StandardCopyOption.REPLACE_EXISTING)      
+
+      String pomFileName = jarFile.fileName.toString().replace('.jar', '.pom')
+      String apiPomFileName = apiFileName.replace('.jar', '.pom')
+      Files.copy(jarFile.resolveSibling(pomFileName), jarFile.resolveSibling(apiPomFileName),
+                 StandardCopyOption.REPLACE_EXISTING)
+    }
+  }
+  project.publishing {
+    publications {
+      apijar(MavenPublication) {
+        from project.components.java
+        artifactId = artifactId + '-api'
+        pom.withXml { XmlProvider xml ->
+          Node root = xml.asNode()
+          root.appendNode('name', project.pluginProperties.extension.name)
+          root.appendNode('description', project.pluginProperties.extension.description)
+        }
+      }
+    }
+  }
+}
+
 integTest {
   // TODO: fix this rest test to not depend on a hardcoded port!
   systemProperty 'tests.rest.blacklist', 'getting_started/10_monitor_cluster_health/*,bulk/10_basic/*'