plugins/completion: move to by-name

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

plugins/default.nix

@@ -1,5 +1,7 @@
 {
   imports = [
+    ./cmp
+
     ./colorschemes/ayu.nix
     ./colorschemes/base16
     ./colorschemes/catppuccin.nix
@@ -22,15 +24,6 @@
     ./colorschemes/tokyonight.nix
     ./colorschemes/vscode.nix
 
-    ./completion/codeium-vim.nix
-    ./completion/codeium-nvim.nix
-    ./completion/copilot-lua.nix
-    ./completion/copilot-vim.nix
-    ./completion/coq.nix
-    ./completion/coq-thirdparty.nix
-    ./completion/lspkind.nix
-    ./completion/cmp
-
     ./filetrees/chadtree.nix
     ./filetrees/neo-tree.nix
     ./filetrees/nvim-tree.nix