Make gitea, gitea-vet future-proof (#18361)

* Make gitea, gitea-vet future-proof

- Ref: https://gitea.com/gitea/gitea-vet/pulls/18

* Correct order

Co-authored-by: zeripath <art27@cantab.net>

CONTRIBUTING.md

@@ -141,8 +141,8 @@ For imports you should use the following format (_without_ the comments)
 ```go
 import (
   // stdlib
-  "encoding/json"
   "fmt"
+  "math"
 
   // local packages
   "code.gitea.io/gitea/models"

modules/json/json.go

@@ -4,6 +4,7 @@
 
 package json
 
+// Allow "encoding/json" import.
 import (
 	"bytes"
 	"encoding/binary"