* guix/import/stackage.scm (<stackage-lts>, <snapshot>)
(<stackage-package>): New record types and JSON mappings.
(lts-info-packages, stackage-package-name)
(stackage-package-version): Remove.
(lts-package-version): Rename 'pkgs-info' to 'packages'; assume
'packages' is a list of <stackage-package>.
(stackage->guix-package): Use 'stackage-lts-packages' instead of
'lts-info-packages'. Rename 'packages-info' to 'packages'.
(latest-lts-release): Likewise.
(stackage-package?): Rename to...
(stackage-lts-package?): ... this. Adjust to new API.
(%stackage-updater)[pred]: Update accordingly.
* tests/lint.scm ("haskell-stackage"): Add "snapshot" entry in JSON
snippet.