[Core] Fix bootstrap stubs load on PHP 7.2 when vendor/ excluded via skip()

[samsonasik]

@TomasVotruba this is to fix issue that only happen in PHP 7.2, which can make error:

"Child process error: PHP Fatal error:  Class 'ReflectionUnionType' not found

while working on PHP 7.3 and PHP 7.4. Reproduced repository :

https://github.com/Micr0mega/rector-issue-7196/runs/6651988104?check_suite_focus=true#step:5:10

/cc @Micr0mega

Fixes rectorphp/rector#7196

[samsonasik]

All checks have passed 🎉 @TomasVotruba it is ready for review.

[TomasVotruba]

Thanks 👍

Please add a reproducer test, so we have it covered for future.

[samsonasik]

I've added e2e test for rector/rector for run under php 7.2 9e99442

[samsonasik]

All checks have passed 🎉 @TomasVotruba I think it is ready.

[samsonasik]

rebased.

[TomasVotruba]

Thank you 👏