Remove the out-projection from kotlinSourceSets in kotlin extension

Gradle versions older than 4.10 could not correctly provide access to
the source set via delegation (i.e. `getting`, `creating` etc.) because
of the out-projection.

Remove the out-projection and expose just
`DomainObjectContainer<KotlinSourceSet>`, as under the hood we can still
use the `DefaultKotlinSourceSet` type for the items.

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -37,18 +37,20 @@ class NewMultiplatformIT : BaseGradleIT() {
     @Test
     fun testLibAndApp() = doTestLibAndApp(
         "sample-lib",
-        "sample-app",
-        gradleVersion
+        "sample-app"
     )
 
     @Test
     fun testLibAndAppWithGradleKotlinDsl() = doTestLibAndApp(
         "sample-lib-gradle-kotlin-dsl",
         "sample-app-gradle-kotlin-dsl",
-        GradleVersionRequired.AtLeast("4.10.2") // Using a SNAPSHOT version in the plugins DSL requires 4.10
+        GradleVersionRequired.AtLeast("4.9") // earlier Gradle versions fail at accessors codegen
     )
 
-    fun doTestLibAndApp(libProjectName: String, appProjectName: String, gradleVersionRequired: GradleVersionRequired) {
+    private fun doTestLibAndApp(
+        libProjectName: String, appProjectName: String,
+        gradleVersionRequired: GradleVersionRequired = gradleVersion
+    ) {
         val libProject = transformProjectWithPluginsDsl(libProjectName, gradleVersionRequired, "new-mpp-lib-and-app")
         val appProject = transformProjectWithPluginsDsl(appProjectName, gradleVersionRequired, "new-mpp-lib-and-app")
         val oldStyleAppProject = Project("sample-old-style-app", gradleVersionRequired, "new-mpp-lib-and-app")
@@ -234,7 +236,9 @@ class NewMultiplatformIT : BaseGradleIT() {
             classesWithoutJava = getFilePathsSet("build/classes")
         }
 
-        gradleBuildScript().modify { it.replace("presets.jvm", "presets.jvmWithJava") }
+        gradleBuildScript().modify {
+            it.replace("presets.jvm", "presets.jvmWithJava").replace("jvm(", "targetFromPreset(presets.jvmWithJava, ")
+        }
 
         projectDir.resolve("src/main/java").apply {
             mkdirs()

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/new-mpp-lib-and-app/sample-lib-gradle-kotlin-dsl/build.gradle.kts

@@ -21,11 +21,11 @@ kotlin {
     macosX64("macos64")
       
     targets.all {
-        mavenPublication {
-            pom.withXml {
+        mavenPublication(Action<MavenPublication> {
+            pom.withXml(Action<XmlProvider> {
                 asNode().appendNode("name", "Sample MPP library")
-            }
-        }
+            })
+        })
     }
 
     sourceSets {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinProjectExtension.kt

@@ -38,9 +38,9 @@ open class KotlinProjectExtension {
     val experimental: ExperimentalExtension
         get() = DslObject(this).extensions.getByType(ExperimentalExtension::class.java)
 
-    var sourceSets: NamedDomainObjectContainer<out KotlinSourceSet>
+    var sourceSets: NamedDomainObjectContainer<KotlinSourceSet>
         @Suppress("UNCHECKED_CAST")
-        get() = DslObject(this).extensions.getByName("sourceSets") as NamedDomainObjectContainer<out KotlinSourceSet>
+        get() = DslObject(this).extensions.getByName("sourceSets") as NamedDomainObjectContainer<KotlinSourceSet>
         internal set(value) {
             DslObject(this).extensions.add("sourceSets", value)
         }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPluginWrapper.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.gradle.plugin
 
+import org.gradle.api.NamedDomainObjectFactory
 import org.gradle.api.Plugin
 import org.gradle.api.Project
 import org.gradle.api.internal.FeaturePreviews
@@ -28,7 +29,6 @@ import org.jetbrains.kotlin.gradle.dsl.*
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMultiplatformPlugin
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinUsages
 import org.jetbrains.kotlin.gradle.plugin.sources.DefaultKotlinSourceSetFactory
-import org.jetbrains.kotlin.gradle.plugin.sources.KotlinSourceSetFactory
 import org.jetbrains.kotlin.gradle.tasks.KOTLIN_COMPILER_EMBEDDABLE
 import org.jetbrains.kotlin.gradle.tasks.KOTLIN_MODULE_GROUP
 import java.io.FileNotFoundException
@@ -44,7 +44,7 @@ abstract class KotlinBasePluginWrapper(
 
     open val projectExtensionClass: KClass<out KotlinProjectExtension> get() = KotlinProjectExtension::class
 
-    internal open fun kotlinSourceSetFactory(project: Project): KotlinSourceSetFactory<out KotlinSourceSet> =
+    internal open fun kotlinSourceSetFactory(project: Project): NamedDomainObjectFactory<KotlinSourceSet> =
         DefaultKotlinSourceSetFactory(project, fileResolver)
 
     override fun apply(project: Project) {
@@ -64,8 +64,8 @@ abstract class KotlinBasePluginWrapper(
         val kotlinGradleBuildServices = KotlinGradleBuildServices.getInstance(project.gradle)
 
         project.createKotlinExtension(projectExtensionClass).apply {
-            fun <T : KotlinSourceSet> kotlinSourceSetContainer(factory: KotlinSourceSetFactory<T>) =
-                project.container(factory.itemClass, factory)
+            fun kotlinSourceSetContainer(factory: NamedDomainObjectFactory<KotlinSourceSet>) =
+                project.container(KotlinSourceSet::class.java, factory)
 
             project.kotlinExtension.sourceSets = kotlinSourceSetContainer(kotlinSourceSetFactory(project))
         }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/sources/KotlinSourceSetFactory.kt

@@ -18,7 +18,7 @@ import java.io.File
 internal abstract class KotlinSourceSetFactory<T : KotlinSourceSet> internal constructor(
     protected val fileResolver: FileResolver,
     protected val project: Project
-) : NamedDomainObjectFactory<T> {
+) : NamedDomainObjectFactory<KotlinSourceSet> {
 
     abstract val itemClass: Class<T>