Lift isRoot extension as it's used in two places.

Author: samdeane

Sources/SwiftFormat/Utilities/FileIterator.swift

@@ -209,17 +209,3 @@ private func inputShouldBeProcessed(at url: URL) -> Bool {
   } while !containingDirectory.isRoot
   return true
 }
-
-fileprivate extension URL {
-  var isRoot: Bool {
-    #if os(Windows)
-    // FIXME: We should call into Windows' native check to check if this path is a root once https://github.com/swiftlang/swift-foundation/issues/976 is fixed.
-    // https://github.com/swiftlang/swift-format/issues/844
-    return self.pathComponents.count <= 1
-    #else
-    // On Linux, we may end up with an string for the path due to https://github.com/swiftlang/swift-foundation/issues/980
-    // TODO: Remove the check for "" once https://github.com/swiftlang/swift-foundation/issues/980 is fixed.
-    return self.path == "/" || self.path == ""
-    #endif
-  }
-}

Sources/SwiftFormat/Utilities/URL+IsRoot.swift

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+extension URL {
+  var isRoot: Bool {
+    #if os(Windows)
+    // FIXME: We should call into Windows' native check to check if this path is a root once https://github.com/swiftlang/swift-foundation/issues/976 is fixed.
+    // https://github.com/swiftlang/swift-format/issues/844
+    return self.pathComponents.count <= 1
+    #else
+    // On Linux, we may end up with an string for the path due to https://github.com/swiftlang/swift-foundation/issues/980
+    // TODO: Remove the check for "" once https://github.com/swiftlang/swift-foundation/issues/980 is fixed.
+    return self.path == "/" || self.path == ""
+    #endif
+  }
+}