plugins/by-name: init

Add support for automatically importing any directories under
`plugins/by-name`.

Includes a validation test, which is run by CI and by the pre-commit hook.

modules/plugins.nix

@@ -1,4 +1,17 @@
-{ ... }:
+{ lib, ... }:
+let
+  inherit (builtins) readDir pathExists;
+  inherit (lib.attrsets) foldlAttrs;
+  inherit (lib.lists) optional optionals;
+  by-name = ../plugins/by-name;
+in
 {
-  imports = [ ../plugins/default.nix ];
+  imports =
+    [ ../plugins ]
+    ++ optionals (pathExists by-name) (
+      foldlAttrs (
+        prev: name: type:
+        prev ++ optional (type == "directory") (by-name + "/${name}")
+      ) [ ] (readDir by-name)
+    );
 }