InstallSchema likes Vendor\Namespace but does not like Vendor\Name\Space

Environment

Magento 2.2

Summary (*)

First, I chose not to report this as a bug because it could just be me not following PSR-4 best practices. So, let's communicate about it first.

When Magento checks for InstallSchema it does not honor the composer-configured PSR-4 root.

Examples (*)

• If I set my composer.json to the config below, it is honored by Composer and Magento. However, it is not honored for setup scripts when magento setup:module is attempting to discover them

  "autoload": {
    "psr-4": {
      "Vendor\\Name\\Space\\": ""
    },
  }
  

• If I set my composer.json to the normal/popular config below, it is honored by Composer and Magento and the scripts are found during setup:upgrade

  "autoload": {
    "psr-4": {
      "Vendor\\Namespace\\": ""
    },
  }
  

Proposed solution

Have Magento's setup:upgrade logic consider the Composer-configured PSR-4 namespace root.

Start at the setup:upgrade entry point and the InstallSchema discovery logic, then compare its namespace handling with the Composer PSR-4 configuration shown in the issue. Done means setup scripts are discovered when the configured PSR-4 root contains nested namespace segments such as Vendor\Name\Space.