home-environment: add home.extraDependencies

This should have the same effect `system.extraDependencies` has in
nixpkgs: adds paths to the runtime closure without installing them
anywhere.

This is useful for preventing garbage collection of packages that are
expensive to rebuild. For example:

- nixpkgs itself suggests using this for `factorio.src`, which requires
  a token to fetch.
- we can use it to address #6157: avoid needlessly rebuilding
  completions for packages that do not have any.

The implementation mirrors nixpkgs: add a file containing the store
paths we want to keep around to the home-manager-generation derivation.

modules/home-environment.nix

@@ -367,6 +367,15 @@ in
       '';
     };
 
+    home.extraDependencies = mkOption {
+      type = types.listOf types.pathInStore;
+      default = [];
+      description = ''
+        A list of paths that should be included in the home
+        closure but generally not visible.
+      '';
+    };
+
     home.path = mkOption {
       internal = true;
       description = "The derivation installing the user packages.";
@@ -773,6 +782,8 @@ in
           "home-manager-generation"
           {
             preferLocalBuild = true;
+            passAsFile = [ "extraDependencies" ];
+            inherit (config.home) extraDependencies;
           }
           ''
             mkdir -p $out
@@ -790,6 +801,8 @@ in
             ln -s ${config.home-files} $out/home-files
             ln -s ${cfg.path} $out/home-path
 
+            cp "$extraDependenciesPath" "$out/extra-dependencies"
+
             ${cfg.extraBuilderCommands}
           '';