[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.

idea/idea-gradle-native/src/org/jetbrains/kotlin/ide/konan/gradle/KotlinNativeGradleConfigurator.kt

@@ -9,7 +9,7 @@ import org.jetbrains.kotlin.ide.konan.hasKotlinNativeRuntimeInScope
 import org.jetbrains.kotlin.idea.configuration.ConfigureKotlinStatus
 import org.jetbrains.kotlin.idea.configuration.KotlinWithGradleConfigurator
 import org.jetbrains.kotlin.idea.configuration.ModuleSourceRootGroup
-import org.jetbrains.kotlin.resolve.konan.platform.KonanPlatform
+import org.jetbrains.kotlin.platform.konan.KonanPlatforms
 
 open class KotlinNativeGradleConfigurator : KotlinWithGradleConfigurator() {
     override fun getKotlinPluginExpression(forKotlinDsl: Boolean): String = ""
@@ -28,7 +28,7 @@ open class KotlinNativeGradleConfigurator : KotlinWithGradleConfigurator() {
 
     override val name: String get() = NAME
 
-    override val targetPlatform get() = KonanPlatform
+    override val targetPlatform get() = KonanPlatforms.defaultKonanPlatform
 
     override val presentableText get() = PRESENTABLE_TEXT
 

idea/idea-gradle-native/tests/org/jetbrains/kotlin/ide/konan/gradle/GradleNativeLibrariesInIDENamingTest.kt

@@ -19,7 +19,7 @@ import org.jetbrains.kotlin.idea.project.platform
 import org.jetbrains.kotlin.idea.util.projectStructure.allModules
 import org.jetbrains.kotlin.konan.library.konanCommonLibraryPath
 import org.jetbrains.kotlin.konan.library.konanPlatformLibraryPath
-import org.jetbrains.kotlin.platform.impl.isKotlinNative
+import org.jetbrains.kotlin.platform.konan.isNative
 import org.jetbrains.plugins.gradle.util.GradleConstants
 import org.junit.Assert.*
 import org.junit.Test
@@ -111,7 +111,7 @@ private fun assertValidModule(module: Module, projectRoot: String) {
         detectLibraryKind(library.getFiles(OrderRootType.CLASSES)) == NativeLibraryKind
     }
 
-    if (module.platform.isKotlinNative) {
+    if (module.platform.isNative()) {
         assertFalse("No Kotlin/Native libraries in $module", nativeLibraries.isEmpty())
         nativeLibraries.forEach { assertValidNativeLibrary(it, projectRoot) }