Make “sublibrary” standard terminology in docs

[ffaf1]

Substitute “internal library” occourrences with “sublibrary”.

Note that:

• I did not touch hcar documentation files, as they are versioned by date.
• I did update file-format-changelog.rst as it was appropriate.

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Template Β: This PR does not modify cabal behaviour (documentation, tests, refactoring, etc.)

Include the following checklist in your PR:

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[andreabedini]

Previous related discussion here #8567
Maybe let's wait to see if there is consensus.

[Mikolaj]

Maybe let's wait to see if there is consensus.

Let's just make sure we make progress.

[mpilgrem]

A couple of things occurred to me on reading these changes.

[mpilgrem]

I am going to merge this and rebase #9370.

[Mikolaj]

@mpilgrem: thanks a lot for the review and the merge. BTW, we tend to use a slower process for the merge to let people take note and voice last minute opinions: https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions.

[ulysses4ever]

We don't merge manually usually. The process is described here: https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions Please, let's try to stick to the process.

[mpilgrem]

Sorry, I did not mean to rush things. As #9370 is closely related - and perhaps where there is more scope for divergent views - perhaps the debate there can cure my haste here.

[Mikolaj]

No harm done. :)