Rework ExampleSubplugin for the new subplugins API

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

@@ -21,14 +21,18 @@ class SubpluginsIT : BaseGradleIT() {
         project.build("compileKotlin", "build") {
             assertSuccessful()
             assertContains("ExampleSubplugin loaded")
+            assertContains("ExampleLegacySubplugin loaded")
             assertContains("Project component registration: exampleValue")
+            assertContains("Project component registration: exampleLegacyValue")
             assertTasksExecuted(":compileKotlin")
         }
 
         project.build("compileKotlin", "build") {
             assertSuccessful()
             assertContains("ExampleSubplugin loaded")
+            assertContains("ExampleLegacySubplugin loaded")
             assertNotContains("Project component registration: exampleValue")
+            assertNotContains("Project component registration: exampleLegacyValue")
             assertTasksUpToDate(":compileKotlin")
         }
     }

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

@@ -10,6 +10,7 @@ buildscript {
 }
 
 apply plugin: "kotlin"
+apply plugin: "kotlin-example-subplugin"
 
 repositories {
     mavenLocal()