Build: Add back hack for client copying client jar pom (#20577)

Gradle appears to have a bug in maven publshing which will not match the
artifactId of a generated pom with the artifact id it puts in the file.
This adds back a copy hack from the original pom file name to the client
pom file name (which we had before #20403 inadvertently
removed it).

buildSrc/src/main/groovy/org/elasticsearch/gradle/plugin/PluginBuildPlugin.groovy

@@ -148,12 +148,17 @@ public class PluginBuildPlugin extends BuildPlugin {
     /** Adds a task to move jar and associated files to a "-client" name. */
     protected static void addClientJarTask(Project project) {
         Task clientJar = project.tasks.create('clientJar')
-        clientJar.dependsOn(project.jar, project.javadocJar, project.sourcesJar)
+        clientJar.dependsOn(project.jar, 'generatePomFileForClientJarPublication', project.javadocJar, project.sourcesJar)
         clientJar.doFirst {
             Path jarFile = project.jar.outputs.files.singleFile.toPath()
             String clientFileName = jarFile.fileName.toString().replace(project.version, "client-${project.version}")
             Files.copy(jarFile, jarFile.resolveSibling(clientFileName), StandardCopyOption.REPLACE_EXISTING)
 
+            String pomFileName = jarFile.fileName.toString().replace('.jar', '.pom')
+            String clientPomFileName = clientFileName.replace('.jar', '.pom')
+            Files.copy(jarFile.resolveSibling(pomFileName), jarFile.resolveSibling(clientPomFileName),
+                    StandardCopyOption.REPLACE_EXISTING)
+
             String sourcesFileName = jarFile.fileName.toString().replace('.jar', '-sources.jar')
             String clientSourcesFileName = clientFileName.replace('.jar', '-sources.jar')
             Files.copy(jarFile.resolveSibling(sourcesFileName), jarFile.resolveSibling(clientSourcesFileName),
@@ -190,7 +195,7 @@ public class PluginBuildPlugin extends BuildPlugin {
             publications {
                 clientJar(MavenPublication) {
                     from project.components.java
-                    artifactId = artifactId + '-client'
+                    artifactId = project.pluginProperties.extension.name + '-client'
                     pom.withXml { XmlProvider xml ->
                         Node root = xml.asNode()
                         root.appendNode('name', project.pluginProperties.extension.name)