Use external metadata

[noughtmare]

There is now a semi-official source of version information about GHC versions and base versions:

https://github.com/haskell/ghcup-metadata/

The stackage FAQ claims there are also repositories for stackage metadata, but I can't find them (https://github.com/commercialhaskell/stackage/issues/7088). If we can find those and use them then this could also fix #545.