Introduce Kotlin variants with metadata variant and metadata dependency

These two will be needed when we publish a platform-specific part of a
MPP library in order to help the consumers of that single module
(rather than the whole MPP library) find the Kotlin metadata.

`KotlinVariantWithMetadataVariant` will be used for publishing with
Gradle metadata enabled, `KotlinVariantWithMetadataDependency` for
publishing without Gradle metadata, instead adding the Kotlin metadata
as a dependency, just like we did in old MPP.

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

@@ -33,47 +33,6 @@ class KotlinSoftwareComponent(
     override fun getName(): String = name
 }
 
-open class KotlinVariant(
-    override val target: KotlinTarget
-) : SoftwareComponentInternal, KotlinTargetComponent {
-    override fun getUsages(): Set<UsageContext> = target.createUsageContexts()
-    override fun getName(): String = target.name
-
-    // This property is declared in the parent class to allow usages to reference it without forcing the subclass to load,
-    // which is needed for compatibility with older Gradle versions
-    internal var publicationDelegate: MavenPublication? = null
-
-    override val publishable: Boolean
-        get() = target.publishable
-}
-
-class KotlinVariantWithCoordinates(
-    target: KotlinTarget
-) : KotlinVariant(target),
-    ComponentWithCoordinates /* Gradle 4.7+ API, don't use with older versions */
-{
-    override fun getCoordinates() = object : ModuleVersionIdentifier {
-        private val project get() = target.project
-
-        private val moduleName: String get() =
-            publicationDelegate?.artifactId ?:
-            "${project.name}-${target.name.toLowerCase()}"
-
-        private val moduleGroup: String get() =
-            publicationDelegate?.groupId ?:
-            project.group.toString()
-
-        override fun getGroup() = moduleGroup
-        override fun getName() = moduleName
-        override fun getVersion() = publicationDelegate?.version ?: project.version.toString()
-
-        override fun getModule(): ModuleIdentifier = object : ModuleIdentifier {
-            override fun getGroup(): String = moduleGroup
-            override fun getName(): String = moduleName
-        }
-    }
-}
-
 // At the moment all KN artifacts have JAVA_API usage.
 // TODO: Replace it with a specific usage
 object NativeUsage {

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

@@ -0,0 +1,88 @@
+/*
+ * 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.gradle.plugin.mpp
+
+import org.gradle.api.artifacts.ModuleDependency
+import org.gradle.api.artifacts.ModuleIdentifier
+import org.gradle.api.artifacts.ModuleVersionIdentifier
+import org.gradle.api.component.ComponentWithCoordinates
+import org.gradle.api.component.ComponentWithVariants
+import org.gradle.api.internal.component.SoftwareComponentInternal
+import org.gradle.api.internal.component.UsageContext
+import org.gradle.api.publish.maven.MavenPublication
+import org.jetbrains.kotlin.gradle.plugin.KotlinTarget
+import org.jetbrains.kotlin.gradle.plugin.KotlinTargetComponent
+
+open class KotlinVariant(
+    override val target: KotlinTarget
+) : SoftwareComponentInternal, KotlinTargetComponent {
+    override fun getUsages(): Set<UsageContext> = target.createUsageContexts()
+    override fun getName(): String = target.name
+
+    // This property is declared in the parent class to allow usages to reference it without forcing the subclass to load,
+    // which is needed for compatibility with older Gradle versions
+    internal var publicationDelegate: MavenPublication? = null
+
+    override val publishable: Boolean
+        get() = target.publishable
+}
+
+open class KotlinVariantWithCoordinates(
+    target: KotlinTarget
+) : KotlinVariant(target),
+    ComponentWithCoordinates /* Gradle 4.7+ API, don't use with older versions */
+{
+    override fun getCoordinates() = object : ModuleVersionIdentifier {
+        private val project get() = target.project
+
+        private val moduleName: String get() =
+            publicationDelegate?.artifactId ?:
+            "${project.name}-${target.name.toLowerCase()}"
+
+        private val moduleGroup: String get() =
+            publicationDelegate?.groupId ?:
+            project.group.toString()
+
+        override fun getGroup() = moduleGroup
+        override fun getName() = moduleName
+        override fun getVersion() = publicationDelegate?.version ?: project.version.toString()
+
+        override fun getModule(): ModuleIdentifier = object : ModuleIdentifier {
+            override fun getGroup(): String = moduleGroup
+            override fun getName(): String = moduleName
+        }
+    }
+}
+
+class KotlinVariantWithMetadataVariant(target: KotlinTarget, private val metadataTarget: KotlinTarget)
+    : KotlinVariantWithCoordinates(target), ComponentWithVariants {
+    override fun getVariants() = setOf(metadataTarget.component)
+}
+
+class KotlinVariantWithMetadataDependency(target: KotlinTarget, private val metadataTarget: KotlinTarget)
+    : KotlinVariantWithCoordinates(target) {
+    override fun getUsages(): Set<UsageContext> = target.createUsageContexts().mapTo(mutableSetOf()) { usageContext ->
+        UsageContextWithAdditionalDependencies(usageContext, setOf(metadataDependency()))
+    }
+
+    private fun metadataDependency(): ModuleDependency {
+        val metadataPublication = (metadataTarget.component as KotlinVariant).publicationDelegate!!
+        val metadataGroupId = metadataPublication.groupId
+        val metadataArtifactId = metadataPublication.artifactId
+        val metadataVersion = metadataPublication.version
+        return target.project.dependencies.module("$metadataGroupId:$metadataArtifactId:$metadataVersion") as ModuleDependency
+    }
+
+    private class UsageContextWithAdditionalDependencies(
+        val parentUsageContext: UsageContext,
+        val additionalDependencies: Set<ModuleDependency>
+    ) : UsageContext by parentUsageContext {
+        override fun getDependencies() = parentUsageContext.dependencies + additionalDependencies
+
+        // FIXME this is a stub for a function that is not present in the Gradle API that we compile against
+        fun getGlobalExcludes(): Set<Any> = emptySet()
+    }
+}