[Gradle] Keep kotlinSourcesJar in Kotlin/JVM projects

After introducing sourcesElements for Multiplatform projects the task
was not registered. As result of an assumption that Java Gradle Plugin
is taking care of sources publication. But in fact some users can
still rely on Kotlin's `kotlinSourceJar` task instead of JGP's
`sourcesJar`. This look inconsistent and is a matter of a different
issue: KT-54207

^KT-36943

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/AbstractKotlinTarget.kt

@@ -133,11 +133,13 @@ abstract class AbstractKotlinTarget(
         classifierPrefix: String? = null,
         sourcesElementsConfigurationName: String = this.sourcesElementsConfigurationName,
     ): PublishArtifact? {
+        val sourcesJarTask = sourcesJarTask(producingCompilation, componentName, artifactNameAppendix)
+
         // If sourcesElements configuration not found, don't create artifact.
-        // This can happen in pure JVM plugin where source publication is delegated to Java Gradle Plugin
+        // This can happen in pure JVM plugin where source publication is delegated to Java Gradle Plugin.
+        // But we still want to have sourcesJarTask be registered
         project.configurations.findByName(sourcesElementsConfigurationName) ?: return null
 
-        val sourcesJarTask = sourcesJarTask(producingCompilation, componentName, artifactNameAppendix)
         val artifact = project.artifacts.add(sourcesElementsConfigurationName, sourcesJarTask) as ConfigurablePublishArtifact
         artifact.classifier = dashSeparatedName(classifierPrefix, "sources")
         return artifact