Decouple GNOME Shell from gnome-session

[jtojnar]

Currently, gnome-session depends on Shell:

https://github.com/NixOS/nixpkgs/blob/7a4a8e008988b64059e7b8f1ee5e30c0102a36e4/pkgs/desktops/gnome-3/core/gnome-session/default.nix#L49

We do not want that as it introduces unnecessary dependencies into Flashback session. Also it will make things like gnome-shell-with-plugins or optional runtime dependencies much more convenient by dropping the need to rebuild.

We would need to include the session in a similar way we already do with flashback

https://github.com/NixOS/nixpkgs/blob/7a4a8e008988b64059e7b8f1ee5e30c0102a36e4/nixos/modules/services/x11/desktop-managers/gnome3.nix#L168-L175

instead of the hardcoded package

https://github.com/NixOS/nixpkgs/blob/7a4a8e008988b64059e7b8f1ee5e30c0102a36e4/nixos/modules/services/x11/desktop-managers/gnome3.nix#L141

We also need to be mindful of env var stripping.

[stale[bot]]

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