To simplify configuring a `KotlinCompilation` with Gradle Kotlin DSL, it
is essential to expose statically-known types where possible.
This commit parametrizes `KotlinCompilation` with its Kotlin options
type (a subtype of `KotlinCommonOptions`) and adds `kotlinOptions` and
`compileKotlinTask` to `KotlinCompilation`.
(minor) Also reduce the visibility of `attachClassesDir` and
`setupPlugins` to internal, since this API is not for external use.
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.
It was added for Android extensions, but now another mechanism is used there
(PackageFragmentProviderExtension), and there were no other implementations of
ExternalDeclarationsProvider in the project
Writing source annotations enables incremental compilation for kapt2.
However they are not needed in bytecode, so we remove them when
copying classes.
# Conflicts:
# compiler/cli/src/org/jetbrains/kotlin/cli/jvm/K2JVMCompiler.kt
# compiler/frontend.java/src/org/jetbrains/kotlin/config/JVMConfigurationKeys.java
Sergey Igushkin