Add cache sweeping for importmap

Lots of this is taken from solidusio/solidus@64de7bd

lib/alchemy/engine.rb

@@ -22,7 +22,16 @@ class Engine < Rails::Engine
 
     initializer "alchemy.importmap" do |app|
       Alchemy.importmap.draw(Engine.root.join("config", "importmap.rb"))
-      app.config.assets.paths << Alchemy::Engine.root.join("package")
+
+      package_path = Engine.root.join("package")
+      app.config.assets.paths << package_path
+
+      if app.config.importmap.sweep_cache
+        Alchemy.importmap.cache_sweeper(watches: package_path)
+        ActiveSupport.on_load(:action_controller_base) do
+          before_action { Alchemy.importmap.cache_sweeper.execute_if_updated }
+        end
+      end
     end
 
     # Gutentag downcases all tags before save