Move differ default to its own file

Signed-off-by: James Sturtevant <jsturtevant@gmail.com>

defaults/defaults_differ_windows.go

@@ -0,0 +1,23 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package defaults
+
+const (
+	// DefaultDiffer will set the default differ for the platform.
+	// This differ should be compatible with the windows snapshotter.
+	DefaultDiffer = "windows"
+)

defaults/defaults_snapshotter_windows.go

@@ -21,5 +21,4 @@ const (
 	// This will be based on the client compilation target, so take that into
 	// account when choosing this value.
 	DefaultSnapshotter = "windows"
-	DefaultDiffer      = "windows"
 )