Tiny refactor of how Cabal handles configure scripts

[Ericson2314]

See each commit for details. There are no public interface changes, just shuffling around private details.

[Mikolaj]

I've restarted doctests CI and now it passes (previously complained about checksums), so probably a random instability.

[Mikolaj]

As far as I can see, this is indeed a guaranteed safe refactoring, so tests are not mandatory. A changelog file would be welcome, though.

[Mikolaj]

Blocking on a changelog file.

[Ericson2314]

I sort of would think this sort of internal-only change would be the type of thing that belongs in the git history but not a change log, but I added a change log entry as requested.

[Mikolaj]

Actually, you are probably right and I was overzealous. This is not even a change of API, because the module is not yet visible. Anyway, it doesn't hurt.

[Mikolaj]

Please feel free to set a merge label.

[ulysses4ever]

@Ericson2314 please, refer to https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions for a description of how a PR is supposed to be merged.

[andreasabel]

@Ericson2314 : Ah, yes, me setting the "merge me" label wasn't instructing you to do a merge commit, but this is a command to Mergify to rebase+merge (with a 2 day delay). No great harm done, but for the sake of a linear history rebase+merge (e.g. in your case with clean commits) or squash+merge are preferred.

[Ericson2314]

Ah sorry about that, yes I did indeed misinterpet the label.