Teach jsonapi:resource generator to respect namespaced models and controllers

[alsemyonov]

Fixes the issue with namespaced models.

rails generate jsonapi:resource Blog::Post

Before

1. generates app/serializables/blog/serializable_post.rb file with SerializableBlog::Post class inside (💣 breaks autoloading).
2. generated controller is app/controllers/blog/posts_controller.rb,
   but config/routes.rb includes:

   resources :blog/posts

   (💣 and breaks everything with Syntax Error)
3. Factory generated by rails generate factory_bot:model Blog::Post is called :blog_post,
   but API request specs try to create(:post) (💣 Factory not registered! )
4. API request specs files put into spec/api/v1/posts/blog/{create,destroy,index,show,update}_spec.rb (← posts/blog? what is that?)
5. API request specs try to send jsonapi_{get,delete,post,put} '/api/v1/posts' (💣 which nothing can handle)

After:

1. generates app/serializables/blog/serializable_post.rb file with Blog::SerializablePost class inside.
2. generated controller is app/controllers/blog/posts_controller.rb,
   and config/routes.rb includes

   namespace :blog do
     resources :posts
   end

3. Factory generated by rails generate factory_bot:model Blog::Post is called :blog_post,
   and API requests specs try to create(:blog_post)
4. API request specs files put into spec/api/v1/blog/posts/{create,destroy,index,show,update}_spec.rb (← blog/posts, according to generated controller)
5. API request specs try to send jsonapi_{get,delete,post,put} '/api/v1/blog/posts' (which is handled by generated Blog::PostsController