plugins/completion: move to by-name

- Move nvim-cmp to plugins/cmp
- Move other completion plugins to plugins/by-name

plugins/completion/cmp/default.nix

@@ -1,90 +0,0 @@
-{
-  lib,
-  helpers,
-  ...
-}:
-let
-  cmpOptions = import ./options { inherit lib helpers; };
-in
-with lib;
-helpers.neovim-plugin.mkNeovimPlugin {
-  name = "cmp";
-  originalName = "nvim-cmp";
-  package = "nvim-cmp";
-
-  maintainers = [ maintainers.GaetanLepage ];
-
-  description = ''
-    ### Completion Source Installation
-
-    If `plugins.cmp.autoEnableSources` is `true` Nixivm will automatically enable the corresponding source
-    plugins. This is the default behavior, but will work only when this option is set to a list.
-
-    If you use a raw lua string or set `plugins.cmp.autoEnableSources` to `false`, you will need to explicitly enable the relevant source plugins in
-    your nixvim configuration.
-
-    #### With auto-enabled sources
-    ```nix
-    plugins.cmp = {
-      autoEnableSources = true;
-      settings.sources = [
-        { name = "nvim_lsp"; }
-        { name = "path"; }
-        { name = "buffer"; }
-      ];
-    };
-    ```
-
-    #### Without auto-enabled sources
-    ```nix
-    plugins = {
-      cmp = {
-        autoEnableSources = false;
-        settings.sources = [
-          { name = "nvim_lsp"; }
-          { name = "path"; }
-          { name = "buffer"; }
-        ];
-      };
-      cmp-nvim-lsp.enable = true;
-      cmp-path.enable = true;
-      cmp-buffer.enable = true;
-    };
-
-    ```
-  '';
-
-  imports = [
-    # Introduced on 2024 February 21
-    # TODO: remove ~June 2024
-    ./deprecations.nix
-    ./auto-enable.nix
-    ./sources
-  ];
-  deprecateExtraOptions = true;
-
-  inherit (cmpOptions) settingsOptions settingsExample;
-  extraOptions = {
-    inherit (cmpOptions) filetype cmdline;
-  };
-
-  callSetup = false;
-  extraConfig = cfg: {
-    extraConfigLua =
-      ''
-        local cmp = require('cmp')
-        cmp.setup(${helpers.toLuaObject cfg.settings})
-
-      ''
-      + (concatStringsSep "\n" (
-        mapAttrsToList (
-          filetype: settings: "cmp.setup.filetype('${filetype}', ${helpers.toLuaObject settings})\n"
-        ) cfg.filetype
-      ))
-      + (concatStringsSep "\n" (
-        mapAttrsToList (
-          cmdtype: settings: "cmp.setup.cmdline('${cmdtype}', ${helpers.toLuaObject settings})\n"
-        ) cfg.cmdline
-      ));
-  };
-}