Move sdk structs to modules/structs (#6905)

* move sdk structs to moduels/structs

* fix tests

* fix fmt

* fix swagger

* fix vendor

modules/structs/repo_refs.go

@@ -0,0 +1,19 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package structs
+
+// Reference represents a Git reference.
+type Reference struct {
+	Ref    string     `json:"ref"`
+	URL    string     `json:"url"`
+	Object *GitObject `json:"object"`
+}
+
+// GitObject represents a Git object.
+type GitObject struct {
+	Type string `json:"type"`
+	SHA  string `json:"sha"`
+	URL  string `json:"url"`
+}