make modules/structs as a spereate go mod (#7127)

modules/structs/attachment.go

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

modules/structs/go.mod

@@ -0,0 +1,3 @@
+module code.gitea.io/gitea/modules/structs
+
+go 1.12