Update all plugins to use plugin/pkg/log (#1694)

* Update all plugins to use plugin/pkg/log

I wish this could have been done with sed. Alas manually changed all
callers to use the new plugin/pkg/log package.

* Error -> Info

* Add docs to debug plugin as well

plugin/debug/debug_test.go

@@ -1,8 +1,6 @@
 package debug
 
 import (
-	"io/ioutil"
-	"log"
 	"testing"
 
 	"github.com/coredns/coredns/core/dnsserver"
@@ -11,8 +9,6 @@ import (
 )
 
 func TestDebug(t *testing.T) {
-	log.SetOutput(ioutil.Discard)
-
 	tests := []struct {
 		input         string
 		shouldErr     bool