Publish Kotlin class outputs as SourceSet outputs for jdt.ls

- Resolves Java <-> Kotlin in VSCode
- Add build/classes/kotlin/{main,test} as SourceSet outputs (builtBy compileKotlin/compileTestKotlin)
- Improves VSCode Java (jdt.ls via Buildship) classpath so Java can resolve Kotlin symbols in same module
- Metadata-only; no change to packaging or runtime

Signed-off-by: Pekka Nikander <pekka.nikander@iki.fi>

Resolves #14355

core-util-jvm/build.gradle.kts

@@ -3,6 +3,8 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+import org.gradle.api.tasks.SourceSetContainer
+
 val signalJavaVersion: JavaVersion by rootProject.extra
 val signalKotlinJvmTarget: String by rootProject.extra
 
@@ -49,6 +51,20 @@ tasks.runKtlintCheckOverMainSourceSet {
   dependsOn(":core-util-jvm:generateMainProtos")
 }
 
+val sourceSets = extensions.getByName("sourceSets") as SourceSetContainer
+sourceSets.named("main") {
+  output.dir(
+    mapOf("builtBy" to tasks.named("compileKotlin")),
+    "$buildDir/classes/kotlin/main"
+  )
+}
+sourceSets.named("test") {
+  output.dir(
+    mapOf("builtBy" to tasks.named("compileTestKotlin")),
+    "$buildDir/classes/kotlin/test"
+  )
+}
+
 dependencies {
   implementation(libs.kotlin.reflect)
   implementation(libs.kotlinx.coroutines.core)

libsignal-service/build.gradle.kts

@@ -3,6 +3,7 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+import org.gradle.api.tasks.SourceSetContainer
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
@@ -41,6 +42,20 @@ tasks.withType<KotlinCompile>().configureEach {
   }
 }
 
+val sourceSets = extensions.getByName("sourceSets") as SourceSetContainer
+sourceSets.named("main") {
+  output.dir(
+    mapOf("builtBy" to tasks.named("compileKotlin")),
+    "$buildDir/classes/kotlin/main"
+  )
+}
+sourceSets.named("test") {
+  output.dir(
+    mapOf("builtBy" to tasks.named("compileTestKotlin")),
+    "$buildDir/classes/kotlin/test"
+  )
+}
+
 afterEvaluate {
   listOf(
     "runKtlintCheckOverMainSourceSet",