chore: Remove //overrides from depot

This mechanism wasn't actually used for anything other than a minor
external compatibility thing for buildGo.nix, which can also be solved
with a function parameter.

This breaks //fun/gemma because it means that the elmPackages used to
build it are no longer reachable from depot. We'll sort this out later.

Change-Id: I1bf2240435e869cdc4e99bdd1a138fdd2e76f96e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2646
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

default.nix

@@ -71,6 +71,7 @@ let
            (node.meta.targets or []))
     else [];
 in fix(self: {
+  __readTree = [];
   config = config self;
 
   # Elevate 'lib' from nixpkgs
@@ -100,10 +101,4 @@ in fix(self: {
 
 # Add local packages as structured by readTree
 // (localPkgs (readTree' self.config))
-
-# Load overrides into the top-level.
-#
-# This can be used to move things from third_party into the top-level, too (such
-# as `lib`).
-// (readTree' { depot = self; pkgs = self.third_party; }) ./overrides
 )

fun/gemma/default.nix

@@ -1,13 +1,12 @@
 { depot, ... }:
 
 let
-  inherit (depot) elmPackages;
   inherit (depot.third_party) cacert iana-etc libredirect stdenv runCommandNoCC writeText;
 
   frontend = stdenv.mkDerivation {
     name = "gemma-frontend.html";
     src = ./frontend;
-    buildInputs = [ cacert iana-etc elmPackages.elm ];
+    buildInputs = [ cacert iana-etc ]; # elmPackages.elm
 
     # The individual Elm packages this requires are not packaged and I
     # can't be bothered to do that now, so lets open the escape hatch:
@@ -47,4 +46,6 @@ in depot.nix.buildLisp.program {
     ./src/gemma.lisp
     injectFrontend
   ];
-}
+# depot does not currently have Gemma's frontend dependencies, thus
+# the build is disabled.
+} // { meta.ci = false; }

overrides/default.nix

@@ -1,13 +0,0 @@
-# This file is used to move things from nested attribute sets to the
-# top-level.
-{ depot, ... }:
-
-{
-  buildGo = depot.nix.buildGo; # TODO(tazjin): remove this
-
-  # These packages must be exposed for compatibility with buildGo.
-  #
-  # Despite buildGo being tracked in this tree, I want it to be possible
-  # for external users to import it with the default nixpkgs layout.
-  inherit (depot.third_party) go ripgrep;
-}

overrides/elmPackages/default.nix

@@ -1,10 +0,0 @@
-# Gemma needs an older version of Elm to be built. Updating it to
-# the newer version is a lot of effort.
-{ pkgs, ... }:
-
-(import (pkgs.fetchFromGitHub {
-  owner = "NixOS";
-  repo = "nixpkgs";
-  rev = "14f9ee66e63077539252f8b4550049381a082518";
-  sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
-}) {}).elmPackages