Add ability to generate content for model classes from sub-folders of Models directory

[KonstantinLapkovsky]

Implement this logic only after covering the current generators with tests in #49

In some cases, we have model classes inside Model directory subfolders

Models
├── Forum
│   ├── Topic
│   │   ├── Media.php
│   │   └── Thread.php
│   |── User.php
|   └── Post.php
├── Media.php
└── User.php

We should be able to generate content for /Media.php model as well as for /Forum/Topic/Media.php.

To generate content for a model inside a subfolder we should prefix a model name with a folder name separated by the slash /. For sample, Forum/Topic/Media.

When we create a new model we should create its parent folders if they do not exist. For example, the console command php artisan make:entity Forum/Notification/Message creates the next structure

Models
├── Forum
│   ├── Notification
│   │   └── Message.php
│   ├── Topic
│   │   ├── Media.php
│   │   └── Thread.php
│   |── User.php
|   └── Post.php
├── Media.php
└── User.php

The console command php artisan make:entity Forum/Topic/Thread --only-nova-resource should create Nova's resource class for Thread model inside Models/Forum/Topic folder.

If a model does not exist we should receive an error message.

[DenTray]

@KonstantinLapkovsky thanks! I believe it would be great to implement it in the next release.