Provide a Hoogle app output

It’s ostensibly easy to get Hoogle working with Nix – in stacklock2nix, just `devShellArgsModifier = args: args // {withHoogle = true;}` will get you a working server … however, it’s always more of a pain to get rid of the final annoyances than I like. E.g., the `default` dev shell’s Hoogle won’t have access to your local packages.

I‘d like to be able to do
``` bash
nix run .#hoogle
```
and have my full `pkgSet` available. I was also thinking the app should specifically run `hoogle server`, since the tests & db generation would already be handled by the existing Nix Haskell infrastructure. However, it’s probably also useful for searching (again, since the hoogle in the default shell won’t have access to the local packages), so maybe it should be `nix run .#hoogle -- server`.

All I should have to do to get that is add the following to my flake.nix `outputs`:
``` nix
apps.hoogle = pkgs.my-haskell-stacklock.hoogleApp;
```
I.e., I should be able to remove `devShellArgsModifier = args: args // {withHoogle = true;}`.