init: Migrate to Go Modules

Moves vendoring over to Go modules.
Fixes issues found by Go Vet in Go 1.16

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

pkg/init/vendor/github.com/containerd/fifo/errors.go

@@ -0,0 +1,30 @@
+/*
+   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 fifo
+
+import (
+	"errors"
+)
+
+var (
+	ErrClosed      = errors.New("fifo closed")
+	ErrCtrlClosed  = errors.New("control of closed fifo")
+	ErrRdFrmWRONLY = errors.New("reading from write-only fifo")
+	ErrReadClosed  = errors.New("reading from a closed fifo")
+	ErrWrToRDONLY  = errors.New("writing to read-only fifo")
+	ErrWriteClosed = errors.New("writing to a closed fifo")
+)