You can annotate modules with the ANN pragma by using the module keyword. For example:
{-# ANN module (Just "A `Maybe String' annotation") #-}
if the topic is Standard package file format, why not agree on e.g. adopting GenericPackageDescription or another similar haskell type (rather than a text-based file) as the standard?
then any format (cabal, yaml, json, ...) may be used as long as a library exists and is maintained for each such format, which parses / produces the format from / to the standard type?