Move diff to core/diff

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

client/diff.go

@@ -18,8 +18,8 @@ package client
 
 import (
 	diffapi "github.com/containerd/containerd/v2/api/services/diff/v1"
-	"github.com/containerd/containerd/v2/diff"
-	"github.com/containerd/containerd/v2/diff/proxy"
+	"github.com/containerd/containerd/v2/core/diff"
+	"github.com/containerd/containerd/v2/core/diff/proxy"
 )
 
 // DiffService handles the computation and application of diffs