Fix applying subplugin from script

#KT-24497 fixed

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

@@ -131,4 +131,13 @@ class SubpluginsIT : BaseGradleIT() {
             assertFileExists("${kotlinClassesDir(subproject = "a/b", sourceSet = "test")}MyTestClass.class")
         }
     }
+
+    @Test
+    fun testAllopenFromScript() {
+        Project("allOpenFromScript").build("build") {
+            assertSuccessful()
+            assertFileExists("${kotlinClassesDir(sourceSet = "main")}MyClass.class")
+            assertFileExists("${kotlinClassesDir(sourceSet = "test")}MyTestClass.class")
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/allOpenFromScript/build.gradle

@@ -0,0 +1 @@
+apply from: './kotlin.gradle'

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/allOpenFromScript/kotlin.gradle

@@ -0,0 +1,26 @@
+buildscript {
+    repositories {
+        mavenLocal()
+        mavenCentral()
+    }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+        classpath "org.jetbrains.kotlin:kotlin-allopen:$kotlin_version"
+    }
+}
+
+apply plugin: org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper
+apply plugin: org.jetbrains.kotlin.allopen.gradle.AllOpenGradleSubplugin
+
+repositories {
+    mavenLocal()
+    mavenCentral()
+}
+
+allOpen {
+    annotation("AllOpen")
+}
+
+dependencies {
+    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/allOpenFromScript/src/main/kotlin/MyClass.kt

@@ -0,0 +1,9 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+@AllOpen
+class MyClass {
+    fun f() = 0
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/allOpenFromScript/src/main/kotlin/annotations.kt

@@ -0,0 +1,6 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+annotation class AllOpen

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/allOpenFromScript/src/test/kotlin/MyTestClass.kt

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+class MyTestClass : MyClass() {
+    override fun f() = 1
+}

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

@@ -666,7 +666,21 @@ private fun SourceSet.clearJavaSrcDirs() {
 
 private fun loadSubplugins(project: Project, kotlinPluginVersion: String): SubpluginEnvironment =
     try {
-        val subplugins = ServiceLoader.load(KotlinGradleSubplugin::class.java, project.buildscript.classLoader)
+        val klass = KotlinGradleSubplugin::class.java
+        val buildscriptClassloader = project.buildscript.classLoader
+        val klassFromBuildscript = try {
+            buildscriptClassloader.loadClass(klass.canonicalName)
+        } catch (e: ClassNotFoundException) {
+            null
+        }
+
+        val classloader = if (klass == klassFromBuildscript) {
+            buildscriptClassloader
+        } else {
+            klass.classLoader
+        }
+
+        val subplugins = ServiceLoader.load(KotlinGradleSubplugin::class.java, classloader)
             .map { @Suppress("UNCHECKED_CAST") (it as KotlinGradleSubplugin<AbstractCompile>) }
 
         SubpluginEnvironment(subplugins, kotlinPluginVersion)