Build: Improve sourcesJar helper

- Add sources from embedded projects
 - Add artifact to sources configuration
 - Use register instead of create

 #KT-30237 Fixed

libraries/reflect/build.gradle.kts

@@ -177,12 +177,17 @@ val relocateCoreSources by task<Copy> {
 
 tasks.getByName("jar").enabled = false
 
-val sourcesJar = sourcesJar(sourceSet = null) {
+val sourcesJar = tasks.register<Jar>("sourcesJar") {
+    archiveClassifier.set("sources")
+
     dependsOn(relocateCoreSources)
     from(relocatedCoreSrc)
     from("$core/reflection.jvm/src")
 }
 
+addArtifact("archives", sourcesJar)
+addArtifact("sources", sourcesJar)
+
 val result by task<Jar> {
     dependsOn(proguard)
     from(zipTree(file(proguardOutput)))