Generalize the plugin package

Remove containerd specific parts of the plugin package to prepare its
move out of the main repository. Separate the plugin registration
singleton into a separate package.

Separating out the plugin package and registration makes it easier to
implement external plugins without creating a dependency loop.

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

plugin/registry/register.go

@@ -0,0 +1,50 @@
+/*
+   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 registry
+
+import (
+	"sync"
+
+	"github.com/containerd/containerd/plugin"
+)
+
+var register = struct {
+	sync.RWMutex
+	r plugin.Registry
+}{}
+
+// Register allows plugins to register
+func Register(r *plugin.Registration) {
+	register.Lock()
+	defer register.Unlock()
+	register.r = register.r.Register(r)
+}
+
+// Reset removes all global registrations
+func Reset() {
+	register.Lock()
+	defer register.Unlock()
+	register.r = nil
+}
+
+// Graph returns an ordered list of registered plugins for initialization.
+// Plugins in disableList specified by id will be disabled.
+func Graph(filter plugin.DisableFilter) []plugin.Registration {
+	register.RLock()
+	defer register.RUnlock()
+	return register.r.Graph(filter)
+}