Fix publishing with Gradle 5.3 (KT-30379)

In Gradle 5.3, there was a change to the internal logic of publishing
`SoftwareComponent`s with the 'maven-publish' plugin, and,
simultaneously, a new public API was introduced for user-defined
software components, see https://github.com/gradle/gradle/pull/8399

Earlier, the 'maven-publish' plugin used the Gradle `Usage` attribute
values to arrange the dependencies of a software component's
`UsageContext`s into the Maven scopes. Now that mechanism doesn't work
and a `SoftwareComponent` must provide an explicit mapping of the
variants to POMs by using the `SoftwareComponentFactory.adhoc` API.

However, the new API lacks some features we need to properly publish an
MPP, such as adding a component to another one as a variant, so we still
create old-style software components under the hood and
then wrap them into the new API (and then wrap the adhoc components once
again).

We also continue to use the old-style components for Kotlin-specific
logic like finding a target in another project that a dependency
resolved to, as the components produced by the new API lack crucial
information that is used in the logic, like references to
`KotlinCompilation`s.

Issue #KT-30379 Fixed

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

@@ -43,7 +43,7 @@ interface KotlinTarget : Named, HasAttributes {
 
     val publishable: Boolean
 
-    val components: Set<KotlinTargetComponent>
+    val components: Set<SoftwareComponent>
 
     fun mavenPublication(action: Closure<Unit>)
     fun mavenPublication(action: Action<MavenPublication>)