Only add exist paths to reloading target

Currently, `factories`, `test/factories` and `spec/factories` are
specified by default as reload watching paths.
In many applications, there are directories which do not exist (perhaps
do not have` spec` if using minitest and do not have` spec` if using
minitest).

In Rails 5 series, if specify a path that does not exist in
`EventedFileUpdateChecker`, its parent is added to the watching path.
As a result, `node_modules` is also included in the watching path, and
unexpectedly many directories and files are included in listen's watching
targets. Also, if symlink is included in `node_modules`, it also causes
warning of listen.

This issue is solved in Rails 6. However, since many applications use
this gem in Rails 5 and below, it is good not to add directories that
do not exist on the gem side.

Ref: rails/rails#32700

lib/factory_bot_rails/railtie.rb

@@ -31,7 +31,7 @@ class Railtie < Rails::Railtie
     def definition_file_paths
       config.factory_bot.definition_file_paths.map do |path|
         Rails.root.join(path)
-      end
+      end.select(&:exist?)
     end
   end
 end

spec/factory_bot_rails/railtie_spec.rb

@@ -54,4 +54,12 @@ def wait_for_rails_to_reload
       sleep 0.01
     end
   end
+
+  describe "default definition paths" do
+    it "include only exist paths" do
+      expect(FactoryBot.definition_file_paths).to eq(
+        [Rails.root.join("factories")],
+      )
+    end
+  end
 end