imp: provide per channel modules paths to make it easy to "backport"

a module. imports = [ ${myChannelModulesPathk}/installer/...

examples/fully-featured/configurations/Morty.host.nix

@@ -1,4 +1,8 @@
-{ ... }: {
+# auto-special args <channelName>ModulesPath for easy backporting of modules
+{ unstableModulesPath, ... }: {
+
+  imports = [ "${unstableModulesPath}/installer/cd-dvd/installation-cd-minimal-new-kernel.nix" ];
+  disabledModules = [ "installer/cd-dvd/installation-cd-minimal-new-kernel.nix" ];
+
   boot.loader.grub.devices = [ "nodev" ];
-  fileSystems."/" = { device = "test"; fsType = "ext4"; };
 }

systemFlake.nix

@@ -30,7 +30,7 @@
 
 let
   inherit (flake-utils-plus.lib) eachSystem patchChannel;
-  inherit (builtins) foldl' mapAttrs removeAttrs attrValues isAttrs isList;
+  inherit (builtins) foldl' mapAttrs removeAttrs attrValues attrNames isAttrs isList;
 
   # set defaults and validate host arguments
   evalHostArgs =
@@ -42,7 +42,16 @@ let
     , extraArgs ? { }
       # These are not part of the module system, so they can be used in `imports` lines without infinite recursion
     , specialArgs ? { }
-    }: { inherit channelName system output builder modules extraArgs specialArgs; };
+    }: {
+      inherit channelName system output builder modules extraArgs;
+      specialArgs = let
+        f = channelName:
+          { "${channelName}ModulesPath" = "${channels.${channelName}.input}/nixos/modules"; };
+        channelModulesPath = map f (attrNames channels);
+        merge = lhs: rhs: lhs // rhs;
+      in
+        (foldl' merge {} channelModulesPath) // specialArgs;
+    };
 
   # recursively merge attribute sets and lists up to a certain depth
   mergeAny = lhs: rhs: