[Backport #1918 to 5.x] Fix getRelativePath in SourceFinder

[github-actions]

Automated backport of #1918 to branch 5.x.

Do not modify this pull request.

getRelativePath method was implemented incorrectly causing exclude directories being not excluded.

Before change:

Runtime:       PHP 8.4.16
Configuration: /Users/darius/Sites/swagger-php/phpunit.xml.dist

Failed asserting that an array does not have the key '/Users/darius/Sites/swagger-php/docs/examples/specs/using-traits/annotations/Decoration/Whistles.php'.
/Users/darius/Sites/swagger-php/tests/SourceFinderTest.php:33

Time: 00:03.667, Memory: 10.00 MB

There was 1 failure:

1) OpenApi\Tests\SourceFinderTest::testExclude
Failed asserting that an array does not have the key '/Users/darius/Sites/swagger-php/docs/examples/specs/using-traits/annotations/Decoration/Whistles.php'.

/Users/darius/Sites/swagger-php/tests/SourceFinderTest.php:33

FAILURES!
Tests: 1, Assertions: 2, Failures: 1.
Process finished with exit code 1

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-1918-to-5.x
git worktree add --checkout .worktree/backport-1918-to-5.x backport-1918-to-5.x
cd .worktree/backport-1918-to-5.x
git reset --hard HEAD^
git cherry-pick -x cf332956e6603fe4c8da6223a98b7ba65e20231e
git push --force-with-lease