* guix/lint.scm (check-haskell-stackage): New procedure.
(%network-dependent-checkers): Add 'haskell-stackage' checker.
* guix/import/hackage.scm (%hackage-url): New variable.
(hackage-source-url, hackage-cabal-url): Use it in place of a
hard-coded string.
* guix/import/stackage.scm (%stackage-url): Make it a parameter.
(stackage-lts-info-fetch): Update accordingly.
* tests/lint.scm ("hackage-stackage"): New test.