Revert "make modules/structs as a spereate go mod (go-gitea#7127)" (go-gitea#7175)

This reverts commit 835b53f.

Author: lunny

modules/structs/attachment.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 package structs // import "code.gitea.io/gitea/modules/structs"
-
 import (
 	"time"
 )