Explicit repository upload (2nd try)

[Kleidukos]

This PR adds option to specify remote repository.

In case the specified repository is not found in the configuration, or not specified when multiple repositories are configured, a helpful message is displayed to the user:

Error: cabal: Cannot determine a specific hackage repository. Please choose
one with --repository-name. Available repositories are: localhost,
hackage.haskell.org

$ cabal run -- cabal get  -R toto Cabal-syntax
Error: cabal: Cannot find chosen repository toto. Available repositories are:
localhost, hackage.haskell.org

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Template Α: This PR modifies behaviour or interface

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.
  • 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 not been added, as I don't know how to test upload/get actions in the test monad. Happy to add them if someone shows me how!