Issue #3293157 by phenaproxima, tedbow: Make everything internal

package_manager/src/Validator/SymlinkValidator.php

...age_manager/src/Validator/WritableFileSystemValidator.php

src/BatchProcessor.php

src/Controller/ReadinessCheckerController.php

src/Controller/UpdateController.php

src/EventSubscriber/ConfigSubscriber.php

src/Form/UpdateReady.php

src/LegacyVersionUtility.php

src/ProjectInfo.php

src/ReleaseChooser.php

src/Routing/RouteSubscriber.php

src/Validation/ReadinessTrait.php

src/Validation/ReadinessValidationManager.php

src/Validator/CronFrequencyValidator.php

src/Validator/PackageManagerReadinessCheck.php

src/Validator/ScaffoldFilePermissionsValidator.php

src/Validator/StagedDatabaseUpdateValidator.php

src/Validator/StagedProjectsValidator.php

src/Validator/VersionPolicy/ForbidDevSnapshot.php

src/Validator/VersionPolicy/ForbidDowngrade.php