Allow a target to have multiple publications, restructure the code

Instead of exposing a single `component` in a target, allow it to have a
set of `components`, then create a Maven publication for each of the
components.

Move `createUsageContexts` to private API since a target that has
multiple publications may need to create the usage contexts for those
publications differently.

Move some of the components technical stuff to interfaces
`DelegatedToPublication` and
`ComponentWithCoordinatesDelegatedToPublication` in order to reuse it
later for Android's components.

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

@@ -13,7 +13,6 @@ import org.gradle.api.Project
 import org.gradle.api.attributes.AttributeContainer
 import org.gradle.api.attributes.HasAttributes
 import org.gradle.api.component.SoftwareComponent
-import org.gradle.api.internal.component.UsageContext
 import org.gradle.api.publish.maven.MavenPublication
 import org.gradle.util.ConfigureUtil
 import org.jetbrains.kotlin.gradle.dsl.KotlinCommonOptions
@@ -21,6 +20,7 @@ import org.jetbrains.kotlin.gradle.dsl.KotlinCommonOptions
 interface KotlinTargetComponent : SoftwareComponent {
     val target: KotlinTarget
     val publishable: Boolean
+    val defaultArtifactId: String
 }
 
 interface KotlinTarget : Named, HasAttributes {
@@ -41,9 +41,7 @@ interface KotlinTarget : Named, HasAttributes {
 
     val publishable: Boolean
 
-    val component: KotlinTargetComponent
-
-    fun createUsageContexts(): Set<UsageContext>
+    val components: Set<KotlinTargetComponent>
 
     fun mavenPublication(action: Closure<Unit>)
     fun mavenPublication(action: Action<MavenPublication>)