Autoload for vendor root folders

[Rus0]

Description (*)

Added a validation for existing autoload file.

Fixed Issues (if relevant)

1. Magento cannot run with custom Composer vendor directories #17753: Magento cannot run with custom Composer vendor directories

Manual testing scenarios (*)

1. Create a temporary directory for vendor data: mkdir /tmp/composer-vendor-dir
2. Create a project directory: mkdir ~/magento-project
3. Set the Composer vendor directory: export COMPOSER_VENDOR_DIR="/tmp/composer-vendor-dir"
4. Create a Magento project: composer create-project --repository-url=https://repo.magento.com/ magento/project-community-edition ~/magento-project
5. Observe successful Composer installation
6. Attempt to access Magento CLI: php bin/magento

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @Rus0. Thank you for your contribution
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[magento-engcom-team]

@Rus0 thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @Rus0. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[hostep]

@magento-engcom-team or @sidolov : the changes in this PR use tabs for indentation instead of spaces, I'm wondering why the static tests didn't catch this?

[sidolov]

@hostep looks like this test disabled for the Travis, I'll check, thank you for the input!