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>
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