PSR-0 autoloader issue

I have trouble instantiating class by path in my PSR-0 autoloaded project. Structure is as follows:

| - src/
|    - Models/
|        - My_New_Model_Migration.php
|        - ...
|    SeederCommand.php
| - vendor/
|     - ...
| composer.json

And Composer autoloader:

"autoload": {
    "psr-0": {
        "" : [
            "src/"
        ]
    }
}

Not to get to much in SeederCommand class, it is basically a Doctrine/migrations class which is supposed to create a migration with up() and down() methods.

Inside the function responsible for generating those I have this part:

if (file_exists(__DIR__ . "/Models/{$model}_Migration.php")) {
    $class = "{$model}_Migration";
    $instantiated = new $class;
    ....
}

When echoing I notice that file indeed does exist, so this part is working as it should. When newing up the class however I am getting an error:

PHP Fatal error: Uncaught Error: Class '/var/www/html/.../src/Models/My_New_Model_Migration.php' not found in /var/www/html/.../src/SeederCommand.php:97

Since paths are correct, I presume the issue must be in way a PSR-0 autoloader works by parsing paths by underscore?

Is there some way around it?

EDIT:

This answer doesn't help me as it explains the 'why' of the autoloader story. I know how both PSR-0 and PSR-4 autoloaders work on the high level. I would like a way around the fact that autoloader wants a directory structure where there is none (and I don't want it introduced in this case).

EDIT2:

SeederCommand class requires autoload file:

require "../vendor/autoload.php"; 

I've tried doing a dump-autoload and renaming class to one without underscores and same thing happens, so I might have done something wrong with autoloading itself.

EDIT3:

I have tried newing up a class I renamed to a non-underscore version. For example newing up MyNewClass works, but My_New_Class throws an error.

You cannot have underscores in your classname, and not have them in the directory structure.

If your class is named Models_MyWhatever_Migration, because you dynamically add the string "MyWhatever" to the classname during migration, that class MUST be placed in src/Models/MyWhatever/Migration.php. You cannot have it in src/Models/MyWhatever_Migration.php.

If you want to keep the underscore as part of the filename, you have to switch to PSR-4 and use namespaces.