Move convert package to services (#22264)

Addition to #22256

The `convert` package relies heavily on different models which is
[disallowed by our definition of
modules](https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md#design-guideline).
This helps to prevent possible import cycles.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/convert/attachment.go

@@ -0,0 +1,30 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package convert
+
+import (
+	repo_model "code.gitea.io/gitea/models/repo"
+	api "code.gitea.io/gitea/modules/structs"
+)
+
+// ToAttachment converts models.Attachment to api.Attachment
+func ToAttachment(a *repo_model.Attachment) *api.Attachment {
+	return &api.Attachment{
+		ID:            a.ID,
+		Name:          a.Name,
+		Created:       a.CreatedUnix.AsTime(),
+		DownloadCount: a.DownloadCount,
+		Size:          a.Size,
+		UUID:          a.UUID,
+		DownloadURL:   a.DownloadURL(),
+	}
+}
+
+func ToAttachments(attachments []*repo_model.Attachment) []*api.Attachment {
+	converted := make([]*api.Attachment, 0, len(attachments))
+	for _, attachment := range attachments {
+		converted = append(converted, ToAttachment(attachment))
+	}
+	return converted
+}