[Gradle] Add deprecated accessor to sourceSets inside KotlinSourceSet

Apparently, using such an approach is a popular case especially for
configuring 'dependsOn' parameter. And we should simplify user migration
 to more restricted DSL.

^KT-64722 Verification Pending

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

@@ -1109,6 +1109,7 @@ public abstract interface class org/jetbrains/kotlin/gradle/plugin/KotlinSourceS
 	public abstract fun getLanguageSettings ()Lorg/jetbrains/kotlin/gradle/plugin/LanguageSettingsBuilder;
 	public abstract fun getResources ()Lorg/gradle/api/file/SourceDirectorySet;
 	public abstract fun getRuntimeOnlyMetadataConfigurationName ()Ljava/lang/String;
+	public abstract fun getSourceSets ()Lorg/gradle/api/NamedDomainObjectContainer;
 	public abstract fun kotlin (Lkotlin/jvm/functions/Function1;)Lorg/gradle/api/file/SourceDirectorySet;
 	public abstract fun kotlin (Lorg/gradle/api/Action;)Lorg/gradle/api/file/SourceDirectorySet;
 	public abstract fun languageSettings (Lkotlin/jvm/functions/Function1;)Lorg/jetbrains/kotlin/gradle/plugin/LanguageSettingsBuilder;

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

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.gradle.plugin
 
 import org.gradle.api.Action
 import org.gradle.api.Named
+import org.gradle.api.NamedDomainObjectContainer
 import org.gradle.api.file.SourceDirectorySet
 import org.jetbrains.kotlin.gradle.dsl.KotlinGradlePluginDsl
 import org.jetbrains.kotlin.tooling.core.HasMutableExtras
@@ -47,4 +48,12 @@ interface KotlinSourceSet : Named, HasProject, HasMutableExtras, HasKotlinDepend
 
     val customSourceFilesExtensions: Iterable<String> // lazy iterable expected
     fun addCustomSourceFilesExtensions(extensions: List<String>) {}
+
+    @Deprecated(
+        "Accessing 'sourceSets' container inside another KotlinSourceSet is deprecated. " +
+                "Consider accessing 'sourceSets' only on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING,
+        replaceWith = ReplaceWith("project.kotlin.sourceSets")
+    )
+    val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
 }

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

@@ -83,8 +83,9 @@ interface KotlinTarget : Named, HasAttributes, HasProject, HasMutableExtras {
 
     @Deprecated(
         "Accessing 'sourceSets' container on the Kotlin target level DSL is deprecated. " +
-                "Consider configuring 'sourceSets' on the Kotlin extension level.",
-        level = DeprecationLevel.WARNING
+                "Consider accessing 'sourceSets' on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING,
+        replaceWith = ReplaceWith("project.kotlin.sourceSets")
     )
     val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
 }

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

@@ -31,8 +31,9 @@ internal interface InternalKotlinTarget : KotlinTarget, HasMutableExtras {
 
     @Deprecated(
         "Accessing 'sourceSets' container on the Kotlin target level DSL is deprecated. " +
-                "Consider configuring 'sourceSets' on the Kotlin extension level.",
-        level = DeprecationLevel.WARNING
+                "Consider accessing 'sourceSets' on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING,
+        replaceWith = ReplaceWith("project.kotlin.sourceSets")
     )
     override val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
         get() = project.extensions.getByType<KotlinProjectExtension>().sourceSets

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/sources/InternalKotlinSourceSet.kt

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlin.gradle.plugin.sources
 
+import org.gradle.api.NamedDomainObjectContainer
+import org.jetbrains.kotlin.gradle.dsl.KotlinProjectExtension
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation
 import org.jetbrains.kotlin.gradle.plugin.KotlinPluginLifecycle
 import org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet
@@ -12,6 +14,7 @@ import org.jetbrains.kotlin.gradle.plugin.await
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMetadataCompilation
 import org.jetbrains.kotlin.gradle.utils.MutableObservableSet
 import org.jetbrains.kotlin.gradle.utils.ObservableSet
+import org.jetbrains.kotlin.gradle.utils.getByType
 
 internal val KotlinSourceSet.internal: InternalKotlinSourceSet
     get() = (this as? InternalKotlinSourceSet) ?: throw IllegalArgumentException(
@@ -26,6 +29,15 @@ internal interface InternalKotlinSourceSet : KotlinSourceSet {
 
     /** Configuration that resolves into sources variants of all Source Set dependencies */
     val dependencySourcesConfigurationName: String
+
+    @Deprecated(
+        "Accessing 'sourceSets' container inside another KotlinSourceSet is deprecated. " +
+                "Consider accessing 'sourceSets' only on the Kotlin extension level.",
+        level = DeprecationLevel.WARNING,
+        replaceWith = ReplaceWith("project.kotlin.sourceSets")
+    )
+    override val sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
+        get() = project.extensions.getByType<KotlinProjectExtension>().sourceSets
 }
 
 internal suspend fun InternalKotlinSourceSet.awaitPlatformCompilations(): Set<KotlinCompilation<*>> {

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

@@ -7,14 +7,30 @@
 
 package org.jetbrains.kotlin.gradle.unitTests
 
+import org.jetbrains.kotlin.gradle.dsl.multiplatformExtension
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation
 import org.jetbrains.kotlin.gradle.plugin.mpp.InternalKotlinCompilation
 import org.jetbrains.kotlin.gradle.util.assertAllImplementationsAlsoImplement
+import org.jetbrains.kotlin.gradle.util.buildProjectWithMPP
 import org.junit.Test
+import kotlin.test.assertEquals
 
 class InternalKotlinSourceSetTest {
     @Test
     fun `test - all implementations of KotlinCompilation - implement InternalKotlinCompilation`() {
         assertAllImplementationsAlsoImplement(KotlinCompilation::class, InternalKotlinCompilation::class)
     }
+
+    @Test
+    fun `sourceSets inside should be equal to extension one`() {
+        val project = buildProjectWithMPP {}
+
+        project.evaluate()
+
+        val projectSourceSets = project.multiplatformExtension.sourceSets
+        project.multiplatformExtension.sourceSets.forEach {
+            @Suppress("DEPRECATION")
+            assertEquals(projectSourceSets, it.sourceSets)
+        }
+    }
 }