Remove app/packs from the autoload paths

Closes #3090.

Author: fxn

lib/webpacker/railtie.rb

@@ -51,4 +51,11 @@ class Webpacker::Engine < ::Rails::Engine
       app.config.javascript_path = Webpacker.config.source_path.relative_path_from(Rails.root.join("app")).to_s
     end
   end
+
+  initializer "webpacker.remove_app_packs_from_the_autoload_paths" do
+    Rails.application.config.before_initialize do
+      source_path = Webpacker.config.source_path.to_s
+      ActiveSupport::Dependencies.autoload_paths.delete(source_path)
+    end
+  end
 end

test/test_app/config/initializers/inspect_autoload_paths.rb

@@ -0,0 +1 @@
+$test_app_autoload_paths_in_initializer = ActiveSupport::Dependencies.autoload_paths

test/webpacker_test.rb

@@ -27,4 +27,8 @@ def dev_server.running?; true; end
       assert Webpacker.inlining_css?
     end
   end
+
+  def test_app_autoload_paths_cleanup
+    assert_empty $test_app_autoload_paths_in_initializer
+  end
 end