When a cabal file can't be found, warn that extra-deps could be the problem too

[MaxGabriel]

• Also: slight refactor to use only concat instead of ++ in the error message
• Also: mention that a package.yaml file would also be accepted
  • The code that throws this is only searching for a .cabal file, but it runs hpack on the directory immediately before that to generate a cabal file, so it's effectively looking for a package.yaml file as well

Closes #3806

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

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• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary. [Not necessary]

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I used stack install to install my changes, then ran that executable:

[mgsloan]

LGTM, thanks! 👍