nixos/plymouth: fix theme dependency resolution in systemd stage 1
nixos/plymouth: fix theme dependency resolution in systemd stage 1

Some plymouth themes use assets of others, like is the case with our
default bgrt depending on spinner. Missing assets would cause the
splashscreen to not render at all in stage 1.

Preliminary dependency resolution code seemed to be broken, and this
should fix it.

Only direct dependencies of selected theme are pulled in.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs