Flake-parts (srid/haskell-flake): how do I get all deps in the devshell?

Now that I solved the most immediate problem with my new flake.nix using flake parts I’ve bumped into the next one.

The flake.nix is the same as in my previous question. I had hoped it would be enough to get a fully functional devshell, but all the dependencies of the package I’m building are missing.

The package I’m building, and which builds perfectly fine using nix build '.#app', is depending on servant:

❯ rg servant tehst.cabal
17:    , servant
18:    , servant-server

but it’s missing in the devshell:

> nix develop
warning: Git tree '/home/elrond/Fabled/tehst' is dirty

(ins)$ ghc-pkg list | rg servant | wc -l

So, what am I missing, how do I get a devshell with all dependencies of the Haskell package?