build: add sourcesJar and javadocJar tasks

These tasks were missing after the migration to the Grails Publish Gradle Plugin.
They are usually added by the Grails Plugin Gradle Plugin, but it is not used in this project.

Using `java.withSourcesJar()` and `java.withJavadocJar()` is not compatible with this build.

Author: matrei

gradle/publish-config.gradle

@@ -2,7 +2,8 @@ import org.grails.gradle.plugin.publishing.GrailsPublishExtension
 
 apply plugin: 'org.grails.grails-publish'
 
-project.extensions.configure(GrailsPublishExtension) {
+extensions.configure(GrailsPublishExtension) {
+    // Explicit `it` is required here
     it.githubSlug = 'grails/grails-data-mapping'
     it.license.name = 'Apache-2.0'
     it.title = findProperty('pomTitle') ?: 'Grails GORM'
@@ -16,3 +17,18 @@ project.extensions.configure(GrailsPublishExtension) {
             'jamesfredley': 'James Fredley'
     ]
 }
+
+// Add sources and javadoc jar tasks
+// java.withSourcesJar() and java.withJavadocJar() is not compatible here with GrailsPublishPlugin
+if (!tasks.findByName('sourcesJar')) {
+    tasks.register('sourcesJar', Jar) {
+        archiveClassifier = 'sources'
+        from sourceSets.main.allSource
+    }
+}
+if (!tasks.findByName('javadocJar')) {
+    tasks.register('javadocJar', Jar) {
+        archiveClassifier = 'javadoc'
+        from javadoc.destinationDir
+    }
+}