Fix sources publication for Gradle plugin as mentioned in gradle/gradle#17760 (comment)

Fixes #310

Author: ileasile

jupyter-lib/kotlin-jupyter-api-gradle-plugin/build.gradle.kts

@@ -48,6 +48,11 @@ val saveVersion by tasks.registering {
     }
 }
 
+java {
+    withSourcesJar()
+    withJavadocJar()
+}
+
 tasks {
     processResources {
         dependsOn(saveVersion)
@@ -59,11 +64,6 @@ tasks {
             events("passed", "skipped", "failed")
         }
     }
-
-    register<Jar>("sourceJar") {
-        archiveClassifier.set("sources")
-        from(sourceSets.named("main").get().allSource)
-    }
 }
 
 val pluginName = "apiGradlePlugin"
@@ -97,12 +97,6 @@ pluginBundle {
 }
 
 publishing {
-    publications {
-        withType<MavenPublication> {
-            artifact(tasks["sourceJar"])
-        }
-    }
-
     repositories {
         (rootProject.findProperty("localPublicationsRepo") as? java.nio.file.Path)?.let {
             maven {