Issue #3254616 by phenaproxima, tedbow: Update Composer Stager to 0.3.0

composer.json

package_manager/package_manager.services.yml

package_manager/src/Event/ExcludedPathsTrait.php

...nager/src/EventSubscriber/ComposerExecutableValidator.php

...e_manager/src/EventSubscriber/ExcludedPathsSubscriber.php

package_manager/src/FileSyncerFactory.php

package_manager/tests/fixtures/fake_site/vendor/.htaccess

...ger/tests/modules/package_manager_bypass/src/Beginner.php

...er/tests/modules/package_manager_bypass/src/Committer.php

...nager/tests/modules/package_manager_bypass/src/Stager.php

package_manager/tests/src/Functional/ExcludedPathsTest.php

...ager/tests/src/Kernel/ComposerExecutableValidatorTest.php

...anager/tests/src/Kernel/ComposerSettingsValidatorTest.php

package_manager/tests/src/Kernel/DiskSpaceValidatorTest.php

..._manager/tests/src/Kernel/ExcludedPathsSubscriberTest.php

...ager/tests/src/Kernel/WritableFileSystemValidatorTest.php

tests/src/Build/UpdateTestBase.php

...ernel/ReadinessValidation/StagedProjectsValidatorTest.php

tests/src/Kernel/UpdaterTest.php