Fixes for review KOTLIN-CR-2599

* Move source artifacts to KotlinVariant classes (since joint Android
 variants may have multiple source artifacts, make it a set)

* Change lowerSpinalCaseName to dashSeparatedName (doesn't transform
 the strings to lower case inside)

* Don't transform the project name to lower case (in the future, it may
 be reasonable to stop transforming the target and component names, too)
 Still transform the target names and the component names.
 Currently, this is the way we don't break existing publications and may
 still reconsider this in the future.

* Add a workaround for a list of GString's assigned to
 publishLibraryVariants

* Check for Android library variants existence in a more strict way,
 report existing variants that are not library variants.

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

@@ -10,6 +10,7 @@ import org.gradle.api.Action
 import org.gradle.api.Named
 import org.gradle.api.NamedDomainObjectContainer
 import org.gradle.api.Project
+import org.gradle.api.artifacts.PublishArtifact
 import org.gradle.api.attributes.AttributeContainer
 import org.gradle.api.attributes.HasAttributes
 import org.gradle.api.component.SoftwareComponent
@@ -21,6 +22,7 @@ interface KotlinTargetComponent : SoftwareComponent {
     val target: KotlinTarget
     val publishable: Boolean
     val defaultArtifactId: String
+    val sourcesArtifacts: Set<PublishArtifact>
 }
 
 interface KotlinTarget : Named, HasAttributes {