treewide: plasma-login-manager fixes (#504074)

nixos/modules/services/desktop-managers/plasma6.nix

@@ -300,7 +300,7 @@ in
     services.orca.enable = mkDefault true;
 
     services.displayManager = {
-      sessionPackages = [ kdePackages.plasma-workspace ];
+      sessionPackages = [ kdePackages.plasma-workspace.sessions ];
       defaultSession = mkDefault "plasma";
     };
     services.displayManager.sddm = {

nixos/modules/services/display-managers/plasma-login-manager.nix

@@ -65,6 +65,16 @@ in
       path = [ cfg.package ];
       wantedBy = [ "graphical.target" ];
       restartIfChanged = false;
+      environment.XDG_DATA_DIRS = lib.mkIf (
+        dmcfg.sessionPackages != [ ]
+      ) "${dmcfg.sessionData.desktops}/share";
+    };
+
+    systemd.user.services.plasma-login = {
+      overrideStrategy = "asDropin";
+      environment.XDG_DATA_DIRS = lib.mkIf (
+        dmcfg.sessionPackages != [ ]
+      ) "${dmcfg.sessionData.desktops}/share";
     };
 
     systemd.defaultUnit = "graphical.target";

pkgs/kde/plasma/plasma-workspace/default.nix

@@ -39,6 +39,13 @@ mkKdeDerivation {
     })
   ];
 
+  outputs = [
+    "out"
+    "dev"
+    "devtools"
+    "sessions"
+  ];
+
   postInstall = ''
     # Prevent patching this shell file, it only is used by sourcing it from /bin/sh.
     chmod -x $out/libexec/plasma-sourceenv.sh
@@ -83,6 +90,9 @@ mkKdeDerivation {
   postFixup = ''
     mkdir -p $out/nix-support
     echo "${lsof} ${xmessage} ${xrdb}" > $out/nix-support/depends
+
+    moveToOutput share/xsessions $sessions
+    moveToOutput share/wayland-sessions $sessions
   '';
 
   passthru.providedSessions = [