Compare paths by their realpath when checking locks

[leongersen]

Use realpath to compare locked files before removal.

Fixes #513

To test:

composer create-project symfony/skeleton example
cd example/vendor/symfony/
rm -rf flex
git clone https://github.com/leongersen/flex --branch issue-513 --single-branch
cd ../../
composer require symfony/test-pack
composer remove symfony/test-pack

[nicolas-grekas]

Thank you @leongersen.