CabalSpecVersion type-class will allow to gather per-spec conditionals.
Currently it's used for selecting parsers / grammatical structure.

Leading (or trailing commas) for CommaFSep/CommaVSep fields,
i.e. fields with mandatory comma are (atm):

- build-depends
- build-tool-depends
- build-tools
- mixins
- pkgconfig-depends
- reexported-modules
- setup-depends