Provide preset-like factory functions for creating targets in new MPP

* Pull the `targets` and `presets` properties of the `kotlin` extension
  up to an interface in `kotlin-gradle-plugin-api`

* Generate the code: we need type-safe statically-typed functions for
  different target type, which seems to be only possible to achieve by
  providing a function per preset.

* Place these functions in the top-level `kotlin` Gradle extension,
  rather than extension functions for `kotlin.targets`:

  - `kotlin.jvm { ... }` will work, while `kotlin.targets.jvm { ... }`
    won't, the extension function needs to be somehow brought into scope

  - reducing the nesting level by one seems to be a good move here

Issue #KT-26389 Fixed

libraries/tools/kotlin-gradle-plugin-dsl-codegen/build.gradle.kts

@@ -0,0 +1,28 @@
+import org.gradle.jvm.tasks.Jar
+
+plugins {
+    kotlin("jvm")
+}
+
+dependencies {
+    compile(gradleApi())
+    compile(project(":kotlin-gradle-plugin-api"))
+    compile(project(":kotlin-native:kotlin-native-utils"))
+}
+
+val generateMppTargetContainerWithPresets by generator(
+    "org.jetbrains.kotlin.generators.gradle.dsl.MppPresetFunctionsCodegenKt",
+    sourceSets["main"]
+)
+
+generateMppTargetContainerWithPresets.run {
+    systemProperty(
+        "org.jetbrains.kotlin.generators.gradle.dsl.outputSourceRoot",
+        project(":kotlin-gradle-plugin").projectDir.resolve("src/main/kotlin").absolutePath
+    )
+}
+
+// Workaround: 'java -jar' refuses to read the original dotted filename on Windows, 'Unable to access jarFile org.jetbrains.kotlin....jar'
+tasks.named<Jar>(generateMppTargetContainerWithPresets.name + "WriteClassPath").configure {
+    archiveName = generateMppTargetContainerWithPresets.name
+}

libraries/tools/kotlin-gradle-plugin-dsl-codegen/src/main/kotlin/org/jetbrains/kotlin/generators/gradle/dsl/codegenUtils.kt

@@ -0,0 +1,34 @@
+/*
+ * 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.
+ */
+
+package org.jetbrains.kotlin.generators.gradle.dsl
+
+internal data class TypeName(val fqName: String, val typeArguments: List<TypeName>)
+
+internal fun typeName(fqName: String, vararg typeArgumentFlatFqNames: String): TypeName {
+    require(typeArgumentFlatFqNames.none { "<" in it }) { "generics won't render to short type names properly, use the full constructor" }
+    return TypeName(
+        fqName,
+        typeArgumentFlatFqNames.map {
+            TypeName(
+                it,
+                emptyList()
+            )
+        })
+}
+
+internal fun TypeName.shortName() = fqName.split(".").last()
+
+internal fun TypeName.packageName() = fqName.substringBeforeLast(".")
+
+internal fun TypeName.renderShort(): String =
+    shortName() + typeArguments.takeIf { it.isNotEmpty() }?.joinToString(", ", "<", ">") { it.renderShort() }.orEmpty()
+
+internal fun TypeName.renderErased(): String =
+    shortName() + typeArguments.takeIf { it.isNotEmpty() }?.joinToString(", ", "<", ">") { "*" }.orEmpty()
+
+internal fun TypeName.collectFqNames(): Set<String> =
+    setOf(fqName) + typeArguments.flatMap { it.collectFqNames() }.toSet()
+

libraries/tools/kotlin-gradle-plugin-dsl-codegen/src/main/kotlin/org/jetbrains/kotlin/generators/gradle/dsl/mppPresetFunctionsCodegen.kt

@@ -0,0 +1,74 @@
+/*
+ * 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.
+ */
+
+package org.jetbrains.kotlin.generators.gradle.dsl
+
+import org.jetbrains.kotlin.gradle.plugin.KotlinTargetsContainerWithPresets
+import java.io.File
+
+fun main() {
+    generateKotlinTargetContainerWithPresetFunctionsInterface()
+}
+
+private val parentInterface = KotlinTargetsContainerWithPresets::class
+
+private val presetsProperty = KotlinTargetsContainerWithPresets::presets.name
+
+private fun generateKotlinTargetContainerWithPresetFunctionsInterface() {
+    // Generate KotlinMutliplatformExtension subclass with member functions for the presets:
+    val functions = allPresetEntries.map {
+        generatePresetFunction(it, presetsProperty, "configureOrCreate")
+    }
+
+    val parentInterfaceName =
+        typeName(parentInterface.java.canonicalName)
+
+    val className =
+        typeName("org.jetbrains.kotlin.gradle.dsl.KotlinTargetContainerWithPresetFunctions")
+
+    val imports = allPresetEntries
+        .flatMap { it.typeNames() }
+        .plus(parentInterfaceName)
+        .filter { it.packageName() != className.packageName() }
+        .flatMap { it.collectFqNames() }
+        .toSortedSet()
+        .joinToString("\n") { "import $it" }
+
+    val generatedCodeWarning = "// DO NOT EDIT MANUALLY! Generated by ${object {}.javaClass.enclosingClass.name}"
+
+    val code = listOf(
+        "package ${className.packageName()}",
+        imports,
+        generatedCodeWarning,
+        "interface ${className.renderShort()} : ${parentInterfaceName.renderShort()} {",
+        functions.joinToString("\n\n") { it.indented(4) },
+        "}"
+    ).joinToString("\n\n")
+
+    val outputSourceRoot = System.getProperties()["org.jetbrains.kotlin.generators.gradle.dsl.outputSourceRoot"]
+    val targetFile = File("$outputSourceRoot/${className.fqName.replace(".", "/")}.kt")
+    targetFile.writeText(code)
+}
+
+private fun generatePresetFunction(
+    presetEntry: KotlinPresetEntry,
+    getPresetsExpression: String,
+    configureOrCreateFunctionName: String
+): String = """
+    fun ${presetEntry.presetName}(
+        name: String = "${presetEntry.presetName}",
+        configure: ${presetEntry.targetType.renderShort()}.() -> Unit = { }
+    ): ${presetEntry.targetType.renderShort()} =
+        $configureOrCreateFunctionName(
+            name,
+            $getPresetsExpression.getByName("${presetEntry.presetName}") as ${presetEntry.presetType.renderErased()},
+            configure
+        )
+""".trimIndent()
+
+private fun String.indented(nSpaces: Int = 4): String {
+    val spaces = String(CharArray(nSpaces) { ' ' })
+    return lines().joinToString("\n") { "$spaces$it"  }
+}

libraries/tools/kotlin-gradle-plugin-dsl-codegen/src/main/kotlin/org/jetbrains/kotlin/generators/gradle/dsl/presetEntries.kt

@@ -0,0 +1,62 @@
+/*
+ * 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.
+ */
+
+package org.jetbrains.kotlin.generators.gradle.dsl
+
+import org.jetbrains.kotlin.konan.target.HostManager
+import org.jetbrains.kotlin.konan.target.presetName
+
+internal class KotlinPresetEntry(
+    val presetName: String,
+    val presetType: TypeName,
+    val targetType: TypeName
+)
+
+internal fun KotlinPresetEntry.typeNames(): Set<TypeName> = setOf(presetType, targetType)
+
+private const val MPP_PACKAGE = "org.jetbrains.kotlin.gradle.plugin.mpp"
+
+private const val KOTLIN_NATIVE_TARGET_PRESET_CLASS_FQNAME = "$MPP_PACKAGE.KotlinNativeTargetPreset"
+private const val KOTLIN_NATIVE_TARGET_CLASS_FQNAME = "$MPP_PACKAGE.KotlinNativeTarget"
+
+private const val KOTLIN_ONLY_TARGET_CLASS_FQNAME = "$MPP_PACKAGE.KotlinOnlyTarget"
+
+internal val jvmPresetEntry = KotlinPresetEntry(
+    "jvm",
+    typeName("$MPP_PACKAGE.KotlinJvmTargetPreset"),
+    typeName(
+        KOTLIN_ONLY_TARGET_CLASS_FQNAME,
+        "$MPP_PACKAGE.KotlinJvmCompilation"
+    )
+)
+
+internal val jsPresetEntry = KotlinPresetEntry(
+    "js",
+    typeName("$MPP_PACKAGE.KotlinJsTargetPreset"),
+    typeName(
+        KOTLIN_ONLY_TARGET_CLASS_FQNAME,
+        "$MPP_PACKAGE.KotlinJsCompilation"
+    )
+)
+
+internal val androidPresetEntry = KotlinPresetEntry(
+    "android",
+    typeName("$MPP_PACKAGE.KotlinAndroidTargetPreset"),
+    typeName("$MPP_PACKAGE.KotlinAndroidTarget")
+)
+
+internal val nativePresetEntries = HostManager().targets.map { (_, target) ->
+    KotlinPresetEntry(
+        target.presetName,
+        typeName(KOTLIN_NATIVE_TARGET_PRESET_CLASS_FQNAME),
+        typeName(KOTLIN_NATIVE_TARGET_CLASS_FQNAME)
+    )
+}
+
+internal val allPresetEntries = listOf(
+    jvmPresetEntry,
+    jsPresetEntry,
+    androidPresetEntry
+) + nativePresetEntries