plugins/completion: move to by-name

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

plugins/cmp/sources/crates-nvim.nix

@@ -0,0 +1,19 @@
+{
+  lib,
+  helpers,
+  config,
+  ...
+}:
+with lib;
+let
+  cfg = config.plugins.crates-nvim;
+in
+{
+  options.plugins.crates-nvim = helpers.neovim-plugin.extraOptionsOptions;
+
+  config = mkIf cfg.enable {
+    extraConfigLua = ''
+      require('crates').setup(${helpers.toLuaObject cfg.extraOptions})
+    '';
+  };
+}