Move conditional task caching setup into the task constructors;

Add 'kotlin.caching.enabled' flag to switch the caching for all tasks;
Rename and change semantics of cache support checks: make two instead
    of one, `isBuildCacheSupported` and `isBuildCacheEnabledForKotlin`;
Remove FileOptionKind entries that are redundant at the moment;
Improvements in BuildCacheIT.kt and other refactoring  suggestions from
    the review
    https://upsource.jetbrains.com/kotlin/review/KOTLIN-CR-1647

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

@@ -21,22 +21,29 @@ import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.compile.AbstractCompile
 import java.io.File
 
-open class SubpluginOption(val key: String, open val value: String)
+open class SubpluginOption(val key: String, val value: String)
 
 class FilesSubpluginOption(
         key: String,
-        val kind: FileOptionKind,
         val files: List<File>,
+        val kind: FilesOptionKind = FilesOptionKind.INTERNAL,
         value: String = files.joinToString(File.pathSeparator) { it.canonicalPath })
     : SubpluginOption(key, value)
 
-class WrapperSubpluginOption(
+class CompositeSubpluginOption(
         key: String,
         value: String,
         val originalOptions: List<SubpluginOption>)
     : SubpluginOption(key, value)
 
-enum class FileOptionKind { INPUT_FILES, CLASSPATH_INPUT, OUTPUT_FILES, OUTPUT_DIRS, INTERNAL }
+/** Defines how the files option should be handled with regard to Gradle model */
+enum class FilesOptionKind {
+    /** The files option is an implementation detail and should not be treated as an input or an output.  */
+    INTERNAL
+
+    // More options might be added when use cases appear for them,
+    // such as output directories, inputs or classpath options.
+}
 
 interface KotlinGradleSubplugin<in KotlinCompile : AbstractCompile> {
     fun isApplicable(project: Project, task: AbstractCompile): Boolean