add caches directory accesssor to FileSystem (#161)

motivation: accessing the caches directory from the FS is a common need and apears in several places in SwiftPM, extracing it out

changes:
* add cachesDirectory:AbsolutePath? to FileSystem protocol
* implement in LocalFileSystem using FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)

Author: tomerd

Sources/TSCBasic/FileSystem.swift

@@ -156,6 +156,9 @@ public protocol FileSystem: class {
 
     /// Get the home directory of current user
     var homeDirectory: AbsolutePath { get }
+    
+    /// Get the caches directory of current user
+    var cachesDirectory: AbsolutePath? { get }
 
     /// Create the given directory.
     func createDirectory(_ path: AbsolutePath) throws
@@ -317,6 +320,10 @@ private class LocalFileSystem: FileSystem {
     var homeDirectory: AbsolutePath {
         return AbsolutePath(NSHomeDirectory())
     }
+    
+    var cachesDirectory: AbsolutePath? {
+        return FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first.flatMap { AbsolutePath($0.absoluteString) }
+    }
 
     func getDirectoryContents(_ path: AbsolutePath) throws -> [String] {
       #if canImport(Darwin)
@@ -622,6 +629,10 @@ public class InMemoryFileSystem: FileSystem {
         // FIXME: Maybe we should allow setting this when creating the fs.
         return AbsolutePath("/home/user")
     }
+    
+    public var cachesDirectory: AbsolutePath? {
+        return self.homeDirectory.appending(component: "caches")
+    }
 
     public func getDirectoryContents(_ path: AbsolutePath) throws -> [String] {
         guard let node = try getNode(path) else {
@@ -870,6 +881,10 @@ public class RerootedFileSystemView: FileSystem {
     public var homeDirectory: AbsolutePath {
         fatalError("homeDirectory on RerootedFileSystemView is not supported.")
     }
+    
+    public var cachesDirectory: AbsolutePath? {
+        fatalError("cachesDirectory on RerootedFileSystemView is not supported.")
+    }
 
     public func getDirectoryContents(_ path: AbsolutePath) throws -> [String] {
         return try underlyingFileSystem.getDirectoryContents(formUnderlyingPath(path))

Tests/TSCUtilityTests/DownloaderTests.swift

@@ -468,6 +468,10 @@ class FailingFileSystem: FileSystem {
     var homeDirectory: AbsolutePath {
         fatalError("unexpected call")
     }
+    
+    var cachesDirectory: AbsolutePath? {
+        fatalError("unexpected call")
+    }
 
     func changeCurrentWorkingDirectory(to path: AbsolutePath) throws {
         fatalError("unexpected call")