plugins/completion: move to by-name

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

tests/test-sources/plugins/cmp/all-sources.nix

@@ -0,0 +1,39 @@
+{ pkgs, ... }:
+{
+  all-sources =
+    { config, ... }:
+    {
+      plugins = {
+        copilot-lua = {
+          enable = true;
+
+          panel.enabled = false;
+          suggestion.enabled = false;
+        };
+
+        cmp = {
+          enable = true;
+          settings.sources =
+            with pkgs.lib;
+            let
+              disabledSources = [
+                # We do not provide the required HF_API_KEY environment variable.
+                "cmp_ai"
+                # Triggers the warning complaining about treesitter highlighting being disabled
+                "otter"
+              ] ++ optional (pkgs.stdenv.hostPlatform.system == "aarch64-linux") "cmp_tabnine";
+            in
+            pipe config.cmpSourcePlugins [
+              # All known source names
+              attrNames
+              # Filter out disabled sources
+              (filter (name: !(elem name disabledSources)))
+              # Convert names to source attributes
+              (map (name: {
+                inherit name;
+              }))
+            ];
+        };
+      };
+    };
+}