Use Python overlay for injecting Plover plugins

[toyboot4e]

Background

Currently, we inject Plover plugins via plover.dependencies:

plover-flake/flake.nix

Lines 79 to 81 in 25ad247

	plover'.overridePythonAttrs (old: {
	dependencies = old.dependencies ++ (f self.ploverPlugins.${pkgs.stdenv.hostPlatform.system});
	});

As in the recent nixpkgs updates, Python overlay sounds like a proper approach. I wonder if it reduces our packaging problems:

• plover: provide wrapPloverExes and withPlugins NixOS/nixpkgs#518828

Check list

• Make up a PR
• Test the behavior on X11, Wayland and macOS
• Ask for review