Use external metadata

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](https://github.com/commercialhaskell/stackage#frequently-asked-questions) 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.

Start by reviewing the GHC/base version metadata at haskell/ghcup-metadata and the linked Stackage FAQ and issue #7088. Determine whether usable Stackage metadata repositories can be found; done means Summoner uses the external version metadata and, if possible, addresses the issue #545 case.

Written by the indexing model from the issue text.