Move client to subpackage

Signed-off-by: Derek McGowan <derek@mcg.dev>

client/snapshotter_default_unix.go

@@ -0,0 +1,26 @@
+//go:build darwin || freebsd || solaris
+
+/*
+   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 client
+
+const (
+	// DefaultSnapshotter will set the default snapshotter for the platform.
+	// This will be based on the client compilation target, so take that into
+	// account when choosing this value.
+	DefaultSnapshotter = "native"
+)