\app
   \models
      \Abc
         -Twitter.php


---------------------------------
contents of app/models/Abc/Twitter.php
---------------------------------

namespace Abc;
class Twitter;
// Other Stuff

--------------------------------
contents of composer.json
--------------------------------

"autoload": {
    "psr-4": {
        "Abc\\": "app/models/Abc"
    }
}

--------------------------------
contents of something that needs Twitter.php
--------------------------------
$twitter = new \Abc\Twitter;