[Gradle, MPP] Publish sources as gradle variants

Now when MPP library is published it will contain "sourcesElements"
variant for each target and one for common code.

Sources variants are published under gradle attributes that were
borrowed from Java Gradle Plugin for consistency reasons.
This makes it compatible with JVM tooling that rely on these attributes.

^KT-36943 Verification Pending

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

@@ -34,6 +34,7 @@ interface KotlinTarget : Named, HasAttributes {
     val defaultConfigurationName: String
     val apiElementsConfigurationName: String
     val runtimeElementsConfigurationName: String
+    val sourcesElementsConfigurationName: String
 
     val publishable: Boolean
 

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinTargetComponent.kt

@@ -13,5 +13,11 @@ interface KotlinTargetComponent : SoftwareComponent {
     val publishable: Boolean
     val publishableOnCurrentHost: Boolean
     val defaultArtifactId: String
+
+    @Deprecated(
+        message = "Sources artifacts are now published as separate variant " +
+                "use target.sourcesElementsConfigurationName to obtain necessary information",
+        replaceWith = ReplaceWith("target.sourcesElementsConfigurationName")
+    )
     val sourcesArtifacts: Set<PublishArtifact>
 }