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/cache/cache_test.go

@@ -1,8 +1,6 @@
 package cache
 
 import (
-	"io/ioutil"
-	"log"
 	"testing"
 	"time"
 
@@ -162,8 +160,6 @@ func TestCache(t *testing.T) {
 
 	c, crr := newTestCache(maxTTL)
 
-	log.SetOutput(ioutil.Discard)
-
 	for _, tc := range cacheTestCases {
 		m := tc.in.Msg()
 		m = cacheMsg(m, tc)