Make builds reproducible

By committing composer.lock, we make sure that dependencies are locked to a given version.
This ensures that builds are reproducible and deterministic.

Also, fixes some phpstan issues, that appeared with the latest version of PHPStan.

.gitignore

@@ -4,6 +4,5 @@
 /tests/resources/generated
 /tests/coverage
 /vendor
-/composer.lock
 /.php-cs-fixer.cache
 /.phpunit.result.cache