Add default implementations for KotlinTarget and KotlinCompilation
(some of them are used in code added by further commits).
Refactor AbstractKotlinPlugin so that parts of its logic can be easily
applied outside the plugin (needed for MPP plugin later).
The single platform Kotlin plugins having Kotlin compiled along with
Java will now create a KotlinCompilation and a KotlinSourceSet per Java
source set or per Android variant.
The build configuration logic will use KotlinCompilation
objects rather than Java or Android plugin entities.
Subplugins API and implementations are updated to use KotlinCompilation.
Sergey Igushkin
dc7d488deb