Issue #3382942: Eliminate dependencies on Composer Stager internals

package_manager/package_manager.services.yml

package_manager/src/FileSyncerFactory.php

package_manager/src/PackageManagerServiceProvider.php

package_manager/tests/src/Kernel/ComposerInspectorTest.php

package_manager/tests/src/Kernel/FileSyncerFactoryTest.php

...manager/tests/src/Kernel/PackageManagerKernelTestBase.php

package_manager/tests/src/Kernel/RsyncValidatorTest.php

package_manager/tests/src/Kernel/ServicesTest.php

package_manager/tests/src/Kernel/StageBaseTest.php

tests/src/Functional/AutomaticUpdatesFunctionalTestBase.php

tests/src/Functional/ComposerStagerOperationFailureTest.php

tests/src/Kernel/ConsoleUpdateStageTest.php

tests/src/Kernel/UpdateStageTest.php

tests/src/Traits/ComposerStagerTestTrait.php