plugins/completion: move to by-name

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

plugins/cmp/sources/_mk-cmp-plugin.nix

@@ -0,0 +1,32 @@
+{
+  lib,
+  helpers,
+  pkgs,
+  ...
+}:
+{
+  pluginName,
+  sourceName,
+  package ? lib.mkPackageOption pkgs [
+    "vimPlugins"
+    pluginName
+  ] { },
+  maintainers ? [ lib.maintainers.GaetanLepage ],
+  imports ? [ ],
+  ...
+}@args:
+helpers.vim-plugin.mkVimPlugin (
+  builtins.removeAttrs args [
+    "pluginName"
+    "sourceName"
+  ]
+  // {
+    inherit package maintainers;
+    name = pluginName;
+
+    imports = imports ++ [
+      # Register the source -> plugin name association
+      { cmpSourcePlugins.${sourceName} = pluginName; }
+    ];
+  }
+)