Allow offline bootstrapping of cabal-install

[sheaf]

This ports to cabal-install the offline bootstrapping logic which was introduced for Hadrian in GHC MR !6315.

This adds a "fetch" command to the bootstrap script, which fetches all the dependency tarballs from Hackage, to be used in an offline build. See bootstrap/README.md for further information.

I tested this locally but would appreciate further testing, perhaps changing the bootstrap CI scripts to separately fetch the archive and then build with the archives.

---

Please include the following checklist in your PR:

• [ ] Patches conform to the coding conventions.
• [ ] Any changes that could be relevant to users have been recorded in the changelog.
• [ ] The documentation has been updated, if necessary.

[Mikolaj]

Thanks a lot. Good point about "changing the bootstrap CI scripts to separately fetch the archive and then build with the archives". That would tick the "test" box required to accept the PR. For the "changelog" box, it's enough to include a tiny file describing the change, as in all other PRs.

[Mikolaj]

Let me rebase to include a workaround to our CI instability and fix the tests.

[Mikolaj]

@mergify rebase

[mergify[bot]]

rebase

✅ Branch has been successfully rebased

[sheaf]

• Added a changelog entry (to cabal-install-3.8.0.20220526.md, let me know if it should be somewhere else).
• Made the bootstrap test use this new functionality.
• Addressed review comments.

[fgaz]

Added a changelog entry (to cabal-install-3.8.0.20220526.md, let me know if it should be somewhere else)

see https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#changelog

[sheaf]

Updated the changelog, grazie Francesco.

[sheaf]

Could we get this merged?

[Mikolaj]

I mean, after the conflict is resolved. :)