Refactor Rebuild monad into writer over sets

[edmundnoble]

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

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I noticed that this seemed like a more natural way for the Rebuild monad to work. Perhaps this even saves us some file watching, I didn't look into it that deeply.

[geekosaur]

This worries me: reverse implies the ordering is significant. (I know it's reversing because it uses : to prepend paths, but that still means the original path order is considered important enough to reconstruct afterward.)

[edmundnoble]

Yeah, this worried me a bit too. I figure this was just out of habit, but I might be wrong.

[Mikolaj]

What's the status of this one (note that #9844 depends on it)? If a design discussion would be prudent before merging one or the other PR, please don't hesitate to open an issue ticket and mark it with the discussion label and lay out the design questions.

[Kleidukos]

@edmundnoble Humble ping to get your attention on this. :)

[edmundnoble]

I will just close this. I'm not sure why I thought #9844 depended on it.