[Gradle] Add deprecated access to sourceSets container inside KotlinTarget

This should keep compatability with user scripts that for any reason tried to configure 'sourceSets' container inside KotlinTarget, but keep such users warned.

^KT-57292 In Progress

libraries/tools/kotlin-gradle-plugin-api/api/kotlin-gradle-plugin-api.api

@@ -1161,6 +1161,7 @@ public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinTarget
 	public abstract fun getPreset ()Lorg/jetbrains/kotlin/gradle/plugin/KotlinTargetPreset;
 	public abstract fun getPublishable ()Z
 	public abstract fun getRuntimeElementsConfigurationName ()Ljava/lang/String;
+	public abstract fun getSourceSets ()Lorg/gradle/api/NamedDomainObjectContainer;
 	public abstract fun getSourcesElementsConfigurationName ()Ljava/lang/String;
 	public abstract fun getTargetName ()Ljava/lang/String;
 	public abstract fun getUseDisambiguationClassifierAsSourceSetNamePrefix ()Z

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinTarget.kt

@@ -17,7 +17,6 @@ import org.gradle.api.publish.maven.MavenPublication
 import org.jetbrains.kotlin.gradle.PRESETS_API_IS_DEPRECATED_MESSAGE
 import org.jetbrains.kotlin.gradle.DeprecatedTargetPresetApi
 import org.jetbrains.kotlin.gradle.InternalKotlinGradlePluginApi
-import org.jetbrains.kotlin.gradle.ExperimentalKotlinGradlePluginApi
 import org.jetbrains.kotlin.gradle.dsl.KotlinCommonCompilerOptions
 import org.jetbrains.kotlin.gradle.dsl.KotlinCommonOptionsDeprecated
 import org.jetbrains.kotlin.gradle.dsl.KotlinGradlePluginDsl
@@ -81,6 +80,13 @@ interface KotlinTarget : Named, HasAttributes, HasProject, HasMutableExtras {
     fun compilerOptions(configure: Action<KotlinCommonCompilerOptions>) {
         throw UnsupportedOperationException("Kotlin target level compiler options DSL is not available in this release!")
     }
+
+    @Deprecated(
+        "Accessing 'sourceSets' container on the Kotlin target level DSL is deprecated. " +
+                "Consider configuring 'sourceSets' on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING
+    )
+    val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
 }
 
 interface KotlinTargetWithTests<E : KotlinExecution.ExecutionSource, T : KotlinTargetTestRun<E>> : KotlinTarget {

libraries/tools/kotlin-gradle-plugin/api/kotlin-gradle-plugin.api

@@ -768,6 +768,7 @@ public class org/jetbrains/kotlin/gradle/plugin/mpp/external/DecoratedExternalKo
 	public final fun getRuntimeElementsConfiguration ()Lorg/gradle/api/artifacts/Configuration;
 	public fun getRuntimeElementsConfigurationName ()Ljava/lang/String;
 	public final fun getRuntimeElementsPublishedConfiguration ()Lorg/gradle/api/artifacts/Configuration;
+	public fun getSourceSets ()Lorg/gradle/api/NamedDomainObjectContainer;
 	public final fun getSourcesElementsConfiguration ()Lorg/gradle/api/artifacts/Configuration;
 	public fun getSourcesElementsConfigurationName ()Ljava/lang/String;
 	public final fun getSourcesElementsPublishedConfiguration ()Lorg/gradle/api/artifacts/Configuration;

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

@@ -5,16 +5,18 @@
 
 package org.jetbrains.kotlin.gradle.plugin.mpp
 
+import org.gradle.api.NamedDomainObjectContainer
 import org.gradle.api.publish.maven.MavenPublication
 import org.jetbrains.kotlin.gradle.InternalKotlinGradlePluginApi
 import org.jetbrains.kotlin.gradle.dsl.KotlinCommonCompilerOptions
+import org.jetbrains.kotlin.gradle.dsl.KotlinProjectExtension
+import org.jetbrains.kotlin.gradle.plugin.*
 import org.jetbrains.kotlin.gradle.plugin.KotlinPluginLifecycle
-import org.jetbrains.kotlin.gradle.plugin.KotlinTarget
-import org.jetbrains.kotlin.gradle.plugin.KotlinTargetComponent
 import org.jetbrains.kotlin.gradle.plugin.await
 import org.jetbrains.kotlin.gradle.plugin.mpp.external.DecoratedExternalKotlinTarget
 import org.jetbrains.kotlin.gradle.plugin.mpp.external.ExternalKotlinTargetImpl
 import org.jetbrains.kotlin.gradle.utils.extrasStoredFuture
+import org.jetbrains.kotlin.gradle.utils.getByType
 import org.jetbrains.kotlin.tooling.core.HasMutableExtras
 
 internal interface InternalKotlinTarget : KotlinTarget, HasMutableExtras {
@@ -24,6 +26,14 @@ internal interface InternalKotlinTarget : KotlinTarget, HasMutableExtras {
     @InternalKotlinGradlePluginApi
     override val components: Set<KotlinTargetSoftwareComponent>
     fun onPublicationCreated(publication: MavenPublication)
+
+    @Deprecated(
+        "Accessing 'sourceSets' container on the Kotlin target level DSL is deprecated. " +
+                "Consider configuring 'sourceSets' on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING
+    )
+    override val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
+        get() = project.extensions.getByType<KotlinProjectExtension>().sourceSets
 }
 
 internal val KotlinTarget.internal: InternalKotlinTarget

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/unitTests/InternalKotlinTargetTest.kt

@@ -7,14 +7,35 @@
 
 package org.jetbrains.kotlin.gradle.unitTests
 
+import org.jetbrains.kotlin.gradle.dsl.multiplatformExtension
 import org.jetbrains.kotlin.gradle.plugin.KotlinTarget
 import org.jetbrains.kotlin.gradle.plugin.mpp.InternalKotlinTarget
 import org.jetbrains.kotlin.gradle.util.assertAllImplementationsAlsoImplement
+import org.jetbrains.kotlin.gradle.util.buildProjectWithMPP
+import org.jetbrains.kotlin.gradle.util.kotlin
 import kotlin.test.Test
+import kotlin.test.assertEquals
 
 class InternalKotlinTargetTest {
     @Test
     fun `test - all implementations of KotlinTarget - implement InternalKotlinTarget`() {
         assertAllImplementationsAlsoImplement(KotlinTarget::class, InternalKotlinTarget::class)
     }
+
+    @Test
+    fun `KotlinTarget sourceSets are equal to extension sourceSets`() {
+        val project = buildProjectWithMPP {
+            kotlin {
+                jvm()
+            }
+        }
+
+        project.evaluate()
+
+        @Suppress("DEPRECATION")
+        assertEquals(
+            project.multiplatformExtension.sourceSets,
+            project.multiplatformExtension.jvm().sourceSets
+        )
+    }
 }