composer.json

{
  "name": "popup-maker/test-plugin",
  "description": "description",
  "minimum-stability": "stable",
  "require": {
    "pimple/pimple": "^3.3"
  },
  "autoload": {
    "psr-4": {
      "DLI\\": "src/"
    }
  },
  "require-dev": {
    "coenjacobs/mozart": "^0.5.1"
  },
  "extra": {
    "mozart": {
      "dep_namespace": "DLI\\Vendor\\",
      "dep_directory": "/src/Vendor/",
      "classmap_directory": "/includes/classes/",
      "classmap_prefix": "DLI_",
      "packages": [
        "pimple/pimple"
      ]
    },
    "delete_vendor_directories": false
  },
  "scripts": {
    "post-install-cmd": [
      "\"vendor/bin/mozart\" compose",
      "composer dump-autoload"
    ],
    "post-update-cmd": [
      "\"vendor/bin/mozart\" compose",
      "composer dump-autoload"
    ]
  }
}

index.php

<?php
/*******************************************************************************
 * Copyright (c) 2020, Code Atlantic LLC.
 ******************************************************************************/

require 'vendor/autoload.php';

use \Pimple\Container;

$container = new Container();