[Layering] Remove direct imports of libc.

 - At least, from the layers above POSIX/Utility.

Author: ddunbar

Sources/Commands/Error.swift

@@ -10,10 +10,10 @@
 
 import Basic
 import PackageModel
-import libc
 
 import enum Utility.ColorWrap
 import enum Utility.Stream
+import func POSIX.exit
 import func Utility.isTTY
 import var Utility.stderr
 

Sources/Commands/Options.swift

@@ -8,10 +8,9 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-import func libc.exit
+import POSIX
 import PackageModel
 import Utility
-import POSIX
 
 public class Options {
     public var chdir: String?

Sources/Commands/SwiftTestTool.swift

@@ -14,7 +14,7 @@ import Basic
 import Utility
 
 import func POSIX.chdir
-import func libc.exit
+import func POSIX.exit
 
 private enum TestError: ErrorProtocol {
     case testsExecutableNotFound

Sources/Commands/build().swift

@@ -8,10 +8,11 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-import func POSIX.getenv
 import PackageModel
 import Utility
-import func libc.exit
+
+import func POSIX.getenv
+import func POSIX.exit
 
 // Builds the default target in the llbuild manifest unless specified.
 public func build(YAMLPath: String, target: String? = nil) throws {

Sources/Get/RawClone.swift

@@ -8,10 +8,10 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-import struct PackageDescription.Version
 import PackageModel
 import Utility
-import libc
+
+import struct PackageDescription.Version
 
 /**
  Initially we clone into a non-final form because we may need to

Sources/POSIX/exit.swift

@@ -0,0 +1,15 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright 2016 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See http://swift.org/LICENSE.txt for license information
+ See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import libc
+
+@noreturn public func exit(_ status: Int32) {
+    libc.exit(status)
+}

Sources/PackageLoading/transmute.swift

@@ -10,7 +10,8 @@
 
 import PackageModel
 import Utility
-import func libc.exit
+
+import func POSIX.exit
 
 /// Load packages into a complete set of modules and products.
 public func transmute(_ rootPackage: Package, externalPackages: [Package]) throws -> (modules: [Module], externalModules: [Module], products: [Product]) {