Introduce Kotlin dependency helpers

 * embeddedKotlinVersion
 * DependencyHandler.kotlinModule(module: String, version: String?)
 * RepositoryHandler.gradleScriptKotlin()

Resolves #94
Resolves #95

build.gradle

@@ -1,7 +1,8 @@
 buildscript {
     ext.kotlinVersion = file('kotlin-version.txt').text.trim()
+    ext.kotlinRepo = 'https://repo.gradle.org/gradle/repo'
     repositories {
-        maven { url 'https://repo.gradle.org/gradle/repo' }
+        maven { url kotlinRepo }
     }
     dependencies {
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
@@ -14,9 +15,18 @@ apply from: 'build.gradle.kts'
 
 // --- Enable automatic generation of API extensions -------------------
 File apiExtensionsOutputDir = file("src/generated/kotlin")
-task generateApiExtensions(type: codegen.GenerateConfigurationExtensions) {
+task generateConfigurationExtensions(type: codegen.GenerateConfigurationExtensions) {
     outputFile = new File(apiExtensionsOutputDir, "org/gradle/script/lang/kotlin/ConfigurationExtensions.kt")
 }
+task generateKotlinDependencyExtensions(type: codegen.GenerateKotlinDependencyExtensions) {
+    outputFile = new File(apiExtensionsOutputDir, "org/gradle/script/lang/kotlin/KotlinDependencyExtensions.kt")
+    embeddedKotlinVersion = kotlinVersion
+    gradleScriptKotlinRepository = kotlinRepo
+}
+task generateApiExtensions {
+    dependsOn generateConfigurationExtensions
+    dependsOn generateKotlinDependencyExtensions
+}
 sourceSets {
     main.kotlin.srcDirs += apiExtensionsOutputDir
 }

buildSrc/src/main/kotlin/codegen/GenerateKotlinDependencyExtensions.kt

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package codegen
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.tasks.Input
+import org.gradle.api.tasks.OutputFile
+import org.gradle.api.tasks.TaskAction
+
+import java.io.File
+
+open class GenerateKotlinDependencyExtensions : DefaultTask() {
+
+    @get:OutputFile
+    var outputFile: File? = null
+
+    @get:Input
+    var embeddedKotlinVersion: String? = null
+
+    @get:Input
+    var gradleScriptKotlinRepository: String? = null
+
+    @TaskAction
+    fun generate() {
+        outputFile!!.writeText(
+            """/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gradle.script.lang.kotlin
+
+import org.gradle.api.artifacts.dsl.DependencyHandler
+import org.gradle.api.artifacts.dsl.RepositoryHandler
+
+import org.gradle.api.artifacts.repositories.ArtifactRepository
+
+/**
+ * The version of the Kotlin compiler embedded in gradle-script-kotlin (currently _${embeddedKotlinVersion}_).
+ */
+val embeddedKotlinVersion = "$embeddedKotlinVersion"
+
+/**
+ * Adds the remote repository containing the Kotlin libraries embedded in gradle-script-kotlin.
+ */
+fun RepositoryHandler.gradleScriptKotlin(): ArtifactRepository =
+    maven { it.setUrl("$gradleScriptKotlinRepository") }
+
+/**
+ * Builds the dependency notation for the named Kotlin [module] at the given [version].
+ *
+ * @param module simple name of the Kotlin module, for example "reflect".
+ * @param version desired version, null implies [embeddedKotlinVersion].
+ */
+fun DependencyHandler.kotlinModule(module: String, version: String? = null): Any =
+    "org.jetbrains.kotlin:kotlin-${'$'}module:${'$'}{version ?: embeddedKotlinVersion}"
+""")
+    }
+}