Refactor some parts of cardano-ledger-conformance to prepare for Dijkstra

[carlostome]

Description

This PR:

• Renames the module SpecTransalate.Core to SpecTranslate.Base, so as to follow the naming structure of formal-ledger-specifications
• Moves SpecTranslate instances
  • from Conway.Base to SpecTranslate.Base, those which are (loosely) Ledger-independent
  • from Conway.Base to SpecTranslate.Core, those which are era-independent
• Removes a stale link to fls in CONTRIBUTING.md

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
• CHANGELOG.md files updated for packages with externally visible changes.
  NOTE: New section is never added with the code changes. (See RELEASING.md).
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary.
  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.

[Soupstraw]

This PR is going to cause a lot of conflicts with #5187

[Soupstraw]

It's better to reimplement this PR once I have merged the big conformance refactoring PR.