* Drop 19.09 support
This is a year old. NixOS doesn't even support the last stable release,
let alone the one before that.
Also cuts our CI size by a third, which is always nice.
* Remove 19.09 compatibility pieces
* cleanSourceHaskell: Add more filters
* cleanSourceHaskell: Add a "name" parameter, split away the filter
* cleanSourceHaskell: Make it compatible with nixpkgs-19.03
* Adjust usage of cleanSourceHaskell in nix-tools
Currently if you override nix-tools using `-I nix-tools` to point to
a local directory that has build output directories like `dist-newstyle`
those directories are hashed and copied to the nix store if they have
changes. This change adds the `cleanHaskellSource` function from
`iohk-nix` and runs it on nix-tools.