[Platform API] Introduce fundamental abstraction of Platform

This is a large commit, which introduces general API for working with
abstraction of Platform.

- Add new abstraction to 'core' - SimplePlatform - which represents
exactly one platform
  - Clients are strongly prohibited to create instances of SimplePlatform
  by hand, instead, corresponding *Platforms abstraction should be used
  (e.g. JvmPlatforms, JsPlatforms, KonanPlatforms)

- Move TargetPlatform to 'core', it represents now a collection of
SimplePlatforms
  - Clients are strongly encouraged to use TargetPlatform
    (not SimplePlatform) in API, to enforce checks for multiplatform

- Provide a helper-extensions to work with TargetPlatform
(in particular, for getting a specific component platform)

- Remove MultiTargetPlatform in favour of TargetPlatform
  - Notably, this commit leaves another widely used duplicated abstraction,
    namely, IdePlatform. For the sake sanity, removal of IdePlatform is
    extracted in the separate commit.

plugins/kapt3/kapt3-compiler/src/org/jetbrains/kotlin/kapt3/Kapt3Plugin.kt

@@ -21,7 +21,6 @@ import com.intellij.openapi.project.Project
 import org.jetbrains.kotlin.analyzer.AnalysisResult
 import org.jetbrains.kotlin.base.kapt3.*
 import org.jetbrains.kotlin.cli.common.CLIConfigurationKeys
-import org.jetbrains.kotlin.cli.common.config.KotlinSourceRoot
 import org.jetbrains.kotlin.cli.common.messages.MessageRenderer
 import org.jetbrains.kotlin.cli.common.messages.PrintingMessageCollector
 import org.jetbrains.kotlin.cli.jvm.config.JavaSourceRoot
@@ -45,9 +44,9 @@ import org.jetbrains.kotlin.kapt3.base.util.KaptLogger
 import org.jetbrains.kotlin.kapt3.util.MessageCollectorBackedKaptLogger
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.resolve.BindingTrace
-import org.jetbrains.kotlin.resolve.TargetPlatform
+import org.jetbrains.kotlin.platform.TargetPlatform
+import org.jetbrains.kotlin.platform.jvm.isJvm
 import org.jetbrains.kotlin.resolve.jvm.extensions.AnalysisHandlerExtension
-import org.jetbrains.kotlin.resolve.jvm.platform.JvmPlatform
 import org.jetbrains.kotlin.utils.decodePluginOptions
 import java.io.ByteArrayInputStream
 import java.io.File
@@ -242,7 +241,7 @@ class Kapt3ComponentRegistrar : ComponentRegistrar {
             platform: TargetPlatform,
             moduleDescriptor: ModuleDescriptor
         ) {
-            if (platform != JvmPlatform) return
+            if (!platform.isJvm()) return
             container.useInstance(KaptAnonymousTypeTransformer())
         }
     }