Add a cabal vendor command - #12522

Fixes #10934. Fixes #10935.

I'll squash commits before applying the merge label if this pull request is approved.

QA notes

In a project with Hackage dependencies, run cabal vendor, add the printed stanza to cabal.project, point remote-repo-cache at an empty directory or remove ~/.cache/cabal/packages, and check that cabal build --offline succeeds.

Check that cabal vendor --dry-run writes nothing, that cabal vendor some-dep vendors only that package and prints active-repositories: :rest, vendored:override, and that running cabal vendor again with the stanza in place succeeds and leaves the directory unchanged.

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)