Migrating PSR-0 to PSR-4 removes a directory level

PSR-0 mapped the entire namespace onto the path, so AppReportBuilder had to live at src/App/Report/Builder.php — the vendor and package name repeated inside a directory that was already the package. PSR-4 lets the prefix map to a directory, and the repetition disappears.

// PSR-0:  "App\": "src/"   =>  src/App/Report/Builder.php
// PSR-4:  "App\": "src/"   =>  src/Report/Builder.php

{
    "autoload": {
        "psr-4": { "App\": "src/" }
    }
}

PSR-0 also translated underscores in class names into directory separators, a PEAR-era compatibility rule that PSR-4 drops — so a class named Report_Builder resolves differently under the two standards. Move one namespace at a time and keep both entries in autoload during the transition; they coexist without complaint.