Use Plugins DSL in Kotlin Gradle scripts

When plugins DSL is used, there is no need to
manually generate typesafe accessors for extensions and
conventions (by running `./gradlew kotlinDslAccessorsSnapshot`).

prepare/android-extensions-compiler-gradle/build.gradle.kts

@@ -3,7 +3,9 @@ import org.gradle.jvm.tasks.Jar
 
 description = "Kotlin Android Extensions Compiler"
 
-apply { plugin("kotlin") }
+plugins {
+    kotlin("jvm")
+}
 
 dependencies {
     compileOnly(project(":compiler:util"))