Refactor CRI fuzzers

pkg/cri/sbserver/cri_fuzzer.go and pkg/cri/server/cri_fuzzer.go were
mostly the same.

This commit merges them together and move the unified fuzzer to
contrib/fuzz again to sort out dependencies. pkg/cri/ shouldn't consume
cmd/.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

pkg/cri/sbserver/fuzz.go

@@ -0,0 +1,35 @@
+//go:build gofuzz
+// +build gofuzz
+
+/*
+   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 sbserver
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/pkg/cri/server"
+	"github.com/containerd/containerd/pkg/cri/store/sandbox"
+)
+
+func SandboxStore(cs server.CRIService) (*sandbox.Store, error) {
+	s, ok := cs.(*criService)
+	if !ok {
+		return nil, fmt.Errorf("%+v is not sbserver.criService", cs)
+	}
+	return s.sandboxStore, nil
+}