nixos module: support NixOS user packages install

When using the NixOS module we cannot guarantee that the Nix store
will be writable during startup. Installing the user packages through
`nix-env -i` will fail in these cases.

This commit adds a NixOS option `home-manager.useUserPackages` that,
when enabled, installs user packages through the NixOS

    users.users.<name?>.packages

option.

Note, when submodule support and external package install is enabled
then the installed packages are not available in `~/.nix-profile`. We
therefore set `home.profileDirectory` directly to the HM profile
packages.

1 files changed, 13 insertions, 0 deletions

diff --git a/modules/misc/submodule-support.nix b/modules/misc/submodule-support.nix
index cffdcb9e95c..d6138c7ccd8 100644
--- a/modules/misc/submodule-support.nix
+++ b/modules/misc/submodule-support.nix
@@ -15,5 +15,18 @@ with lib;
         in, for example, NixOS or nix-darwin.
       '';
     };
+
+    externalPackageInstall = mkOption {
+      type = types.bool;
+      default = false;
+      internal = true;
+      description = ''
+        Whether the packages of <option>home.packages</option> are
+        installed separately from the Home Manager activation script.
+        In NixOS, for example, this may be accomplished by installing
+        the packages through
+        <option>users.users.&lt;name?&gt;.packages</option>.
+      '';
+    };
   };
 }