Using isolated classloader again, but as a singleton; regular compiler jar instead of embeddable; more logging

(cherry picked from commit 6801d8a)

libraries/tools/kotlin-gradle-plugin-core/pom.xml

@@ -44,7 +44,7 @@
         </dependency>
         <dependency>
             <groupId>org.jetbrains.kotlin</groupId>
-            <artifactId>kotlin-compiler-embeddable</artifactId>
+            <artifactId>kotlin-compiler</artifactId>
             <version>${project.version}</version>
         </dependency>
         <dependency>

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

@@ -35,7 +35,9 @@ abstract class KotlinBasePluginWrapper: Plugin<Project> {
         project.getExtensions().getExtraProperties()?.set("kotlin.gradle.plugin.version", kotlinPluginVersion)
 
         if (pluginClassLoader == null)
-            pluginClassLoader = createPluginClassLoader(kotlinPluginVersion, sourceBuildScript)
+            pluginClassLoader = createPluginIsolatedClassLoader(kotlinPluginVersion, sourceBuildScript)
+        else
+            log.kotlinDebug("Reusing classloader from previous run")
         val plugin = getPlugin(pluginClassLoader!!, sourceBuildScript)
         plugin.apply(project)