Here are hpack package.yaml files for the other hledger cabal files.
These remove a lot of human-error-prone duplication.
They are not used yet as hpack isn't quite mature enough -
when it supports flags and benchmarks we will probably switch.
hakyll-std's cabal file is now generated by hpack from an easier yaml
config file, which looks like a valuable timesaver. "make gencabalfiles"
will regenerate this cabal file (and soon the others) when needed.