Create version specific manifest for Swift 4.2 and update Package.swift to swift-tools-version:5.0 (#2737)

* Update swiftLanguageVersions to swift-tools-version:4.2

This API changed from taking [Int] to [SwiftVersion] in PackageDescription API Version 4.2. For more information, see https://github.com/apple/swift-package-manager/blob/master/Documentation/PackageDescriptionV4_2.md#swift-language-version.

* Rename Package.swift to Package@swift-4.2.swift

Upgrade Package.swift to swift-tools-version:5.0

Add platforms definition to Package.swift

* Add platform specifications for iOS, tvOS, and watchOS

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:4.2
+// swift-tools-version:5.0
 //
 //  Package.swift
 //
@@ -27,6 +27,12 @@ import PackageDescription
 
 let package = Package(
     name: "Alamofire",
+    platforms: [
+        .macOS(.v10_12),
+        .iOS(.v10),
+        .tvOS(.v10),
+        .watchOS(.v3)
+    ],
     products: [
         .library(
             name: "Alamofire",
@@ -37,5 +43,5 @@ let package = Package(
             name: "Alamofire",
             path: "Source")
     ],
-    swiftLanguageVersions: [.v3, .v4]
+    swiftLanguageVersions: [.v4]
 )

Package@swift-4.2.swift

@@ -0,0 +1,41 @@
+// swift-tools-version:4.2
+//
+//  Package.swift
+//
+//  Copyright (c) 2014-2018 Alamofire Software Foundation (http://alamofire.org/)
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in
+//  all copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+//  THE SOFTWARE.
+//
+
+import PackageDescription
+
+let package = Package(
+    name: "Alamofire",
+    products: [
+        .library(
+            name: "Alamofire",
+            targets: ["Alamofire"])
+    ],
+    targets: [
+        .target(
+            name: "Alamofire",
+            path: "Source")
+    ],
+    swiftLanguageVersions: [.v3, .v4]
+)