nixos/cosmic: move absolutely necessary packages under corePkgs

The packages without which the COSMIC DE doesn't start at all, or has a
high probability of not initializing are moved in the `corePkgs` list of
packages. This clarifies what packages are not to be excluded when the
user posts an issue about COSMIC crashing with either of the core
packages excluded and inform the user.

nixos/modules/services/desktop-managers/cosmic.nix

@@ -13,6 +13,32 @@
 
 let
   cfg = config.services.desktopManager.cosmic;
+  # **ONLY ADD PACKAGES WITHOUT WHICH COSMIC CRASHES, NOTHING ELSE**
+  corePkgs =
+    with pkgs;
+    [
+      cosmic-applets
+      cosmic-applibrary
+      cosmic-bg
+      cosmic-comp
+      cosmic-files
+      config.services.displayManager.cosmic-greeter.package
+      cosmic-idle
+      cosmic-launcher
+      cosmic-notifications
+      cosmic-osd
+      cosmic-panel
+      cosmic-session
+      cosmic-settings
+      cosmic-settings-daemon
+      cosmic-workspaces-epoch
+    ]
+    ++ lib.optionals cfg.xwayland.enable [
+      # Why would you want to enable XWayland but exclude the package
+      # providing XWayland support? Doesn't make sense. Add `xwayland` to the
+      # `corePkgs` list.
+      xwayland
+    ];
 in
 {
   meta.maintainers = lib.teams.cosmic.members;
@@ -41,44 +67,30 @@ in
       "/share/cosmic"
     ];
     environment.systemPackages = utils.removePackagesByName (
-      with pkgs;
-      [
-        adwaita-icon-theme
-        alsa-utils
-        cosmic-applets
-        cosmic-applibrary
-        cosmic-bg
-        cosmic-comp
-        cosmic-edit
-        cosmic-files
-        config.services.displayManager.cosmic-greeter.package
-        cosmic-icons
-        cosmic-idle
-        cosmic-launcher
-        cosmic-notifications
-        cosmic-osd
-        cosmic-panel
-        cosmic-player
-        cosmic-randr
-        cosmic-screenshot
-        cosmic-session
-        cosmic-settings
-        cosmic-settings-daemon
-        cosmic-term
-        cosmic-wallpapers
-        cosmic-workspaces-epoch
-        hicolor-icon-theme
-        playerctl
-        pop-icon-theme
-        pop-launcher
-        xdg-user-dirs
-      ]
-      ++ lib.optionals cfg.xwayland.enable [
-        xwayland
-      ]
-      ++ lib.optionals config.services.flatpak.enable [
-        cosmic-store
-      ]
+      corePkgs
+      ++ (
+        with pkgs;
+        [
+          adwaita-icon-theme
+          alsa-utils
+          cosmic-edit
+          cosmic-icons
+          cosmic-player
+          cosmic-randr
+          cosmic-screenshot
+          cosmic-term
+          cosmic-wallpapers
+          hicolor-icon-theme
+          playerctl
+          pop-icon-theme
+          pop-launcher
+          xdg-user-dirs
+        ]
+        ++ lib.optionals config.services.flatpak.enable [
+          # User may have Flatpaks enabled but might not want the `cosmic-store` package.
+          cosmic-store
+        ]
+      )
     ) config.environment.cosmic.excludePackages;
 
     # Distro-wide defaults for graphical sessions