Investigate "cabal-plan" tool as an alternative to cabal-dependency-licenses

[jtdaugherty]

The cabal-dependency-licenses tool is unmaintained by its upstream maintainer. We maintain a fork, but keeping it working is a pain. While the tool does what we need, the upstream maintainer of the tool suggests using cabal-plan instead. This ticket is a reminder to investigate whether it will do what we need (easily generate a report of all licenses for our transitive dependency set).