[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.

compiler/frontend.common/src/org/jetbrains/kotlin/analyzer/ModuleInfo.kt

@@ -8,7 +8,7 @@ package org.jetbrains.kotlin.analyzer
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.resolve.PlatformDependentCompilerServices
-import org.jetbrains.kotlin.resolve.TargetPlatform
+import org.jetbrains.kotlin.platform.TargetPlatform
 
 
 interface ModuleInfo {

compiler/frontend.common/src/org/jetbrains/kotlin/resolve/TargetPlatform.kt

@@ -1,11 +0,0 @@
-/*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
- * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
- */
-
-package org.jetbrains.kotlin.resolve
-
-abstract class TargetPlatform(val platformName: String) {
-    override fun toString() = platformName
-    abstract val platform: MultiTargetPlatform
-}