Issue #3258065 by phenaproxima: Create package_manager Validator namespace

package_manager/package_manager.services.yml

...age_manager/src/Validator/ComposerExecutableValidator.php

package_manager/src/Validator/ComposerSettingsValidator.php

package_manager/src/Validator/DiskSpaceValidator.php

package_manager/src/Validator/LockFileValidator.php

package_manager/src/Validator/PendingUpdatesValidator.php

...ger/src/Validator/PreOperationStageValidatorInterface.php

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

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

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

package_manager/tests/src/Kernel/DiskSpaceValidatorTest.php

package_manager/tests/src/Kernel/LockFileValidatorTest.php

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

..._manager/tests/src/Kernel/PendingUpdatesValidatorTest.php

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

src/Validator/PackageManagerReadinessCheck.php

...ReadinessValidation/PackageManagerReadinessChecksTest.php