Reorganize and update nixpkgs upstreams

This patch does a few things:

1. Switches from nixpkgs-unstable to nixos-unstable{,-small}, simply because
   nixpkgs-unstable is not in cache.nixos.org, but nixos-unstable is, and -small
   is the same but requires all tests to pass. So we should prefer
   nixos-unstable-small, whenever possible.

2. Reorganizes the nixpkgs import code such that Nixpkgs.nix returns an attrset
   of all the nixpkgs that I want to use, rather than putting other nixpkgs
   branches into the main one as an overlay. This is much simpler and explicit,
   but it meant I had to change a lot of usages throughtout the nix codebase.

3. As a consequence of 2, moves the overlays into separate files so they can be
   re-used across nixpkgs branches.

1 files changed, 38 insertions, 42 deletions

diff --git a/Biz/Bild/Nixpkgs.nix b/Biz/Bild/Nixpkgs.nix
index 8cb3448..f0e8698 100644
--- a/Biz/Bild/Nixpkgs.nix
+++ b/Biz/Bild/Nixpkgs.nix
@@ -2,54 +2,50 @@ let
   sources = import ./Sources.nix { sourcesFile = ./Sources.json; };
 
   config = {
+    allowAliases = true;
     allowBroken = true;
     allowUnfree = true;
+    checkMeta = true;
     cudaSupport = true;
   };
 
   system = __currentSystem;
 
-in import sources.nixos-23_05 {
-  inherit system config;
-  overlays = [
-    (_: _: { inherit sources; })
-
-    # add some functions
-    (_: old: {
-      # Given a generic `builder`, will generate an attrset for all the packages
-      # pinned by `deps` with `builder` applied to the package. This attrset can
-      # then be merged with the rest of the packages in the set as part of an
-      # overlay or overrides.
-      overridePinnedDeps = builder:
-        old.lib.genAttrs (builtins.attrNames sources) builder;
-
-      # Modifies a derivation with our source and version, keeping old build
-      # rules. This will fail if build steps have changed, or if no build
-      # rules are available upstream.
-      overrideSource = depName:
-        if old ? "${depName}" && old.${depName} ? overrideAttrs then
-          old.${depName}.overrideAttrs (attrs:
-            attrs // rec {
-              version = sources.${depName}.version or sources.${depName}.rev;
-              src = sources.${depName};
-            })
-        else
-          null;
-    })
-
-    # override pinned deps with our sources, this must come before other
-    # package overlays, because of the 'null' above
-    (_: pkgs: pkgs.overridePinnedDeps pkgs.overrideSource)
-
-    # add other nixpkgs distributions
-    (_: _: {
-      unstable = import sources.nixpkgs-unstable { inherit system config; };
-      nixos-23_11 = import sources.nixos-23_11 { inherit system config; };
-    })
-
-    # add our hand-written derivations
-    (import ./Deps.nix)
-    (_: pkgs: { niv = import pkgs.sources.niv { }; })
-  ];
+in {
+  nixos-23_05 = import sources.nixos-23_05 {
+    inherit system config;
+    overlays = [
+      (_: _: { inherit sources; })
+
+      (import ./Functions.nix)
+
+      # override pinned deps with our sources, this must come before other
+      # package overlays, because of the 'null' from 'overrideSource'
+      (_: pkgs: pkgs.overridePinnedDeps pkgs.overrideSource)
+
+      (import ./Deps.nix)
+      (import ./Python.nix)
+      (import ./Haskell.nix)
+    ];
+  };
+
+  nixos-23_11 = import sources.nixos-23_11 {
+    inherit system config;
+    overlays = [
+      (_: _: { inherit sources; })
+      (import ./Functions.nix)
+      (import ./Deps.nix)
+    ];
+  };
+
+  nixos-unstable-small = import sources.nixos-unstable-small {
+    inherit system config;
+    overlays = [
+      (_: _: { inherit sources; })
+      (import ./Functions.nix)
+      (import ./Deps.nix)
+    ];
+  };
+
 }