Check if the path given is already an absolute path

[michelbrito]

Description

The Magento\Framework\Filesystem\Driver\File::isFile function returns false, because it concatenates the symlink path with the absolute path in getAbsolutePath.

Fixed Issues

1. Invalid template files for every vendor templates #8368: Invalid template files for every vendor templates

Manual testing scenarios

1. Put your vendor folder in an other directory (e.g., in /srv/magento/vendor).
2. Create a symbolic link for vendor to its new path (e.g. ln -s /srv/magento/vendor /var/www/vendor).
3. Try to load any website page (e.g. http://example.com/).

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-cicd2]

All committers have signed the CLA.

[orlangur]

https://github.com/magento/magento2/blob/2.2-develop/app/etc/vendor_path.php exists to customize vendor path, why symlinking is needed?

Proposed implementation opens a huge security breach as current implementation is written in such a way that no file must be accessed outside base path.

[sidolov]

@michelbrito , I am closing this PR now due to inactivity.
Please reopen and update if you wish to continue.
Thank you for collaboration