fix: code review alt repository (go-gitea#6663)

I was not able to properly review this specific file from #6351 in time.

- Remove unnecessary second Close call to xz writer.
- Removed unused values.
- Partition packages by architecture in the first loop to reduce the
amount of iterations.
- Don't compile regex, use `path.Split`.
- Pass structs around.
- Remove usage of bytes.buffer when there's no buffering.
- Use a better data structure for building package lists.

TL;DR no functional changes.

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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6663
Reviewed-by: 0ko <[email protected]>
Co-authored-by: Gusted <[email protected]>
Co-committed-by: Gusted <[email protected]>

Author: Gusted