Issue #3246638 by phenaproxima, tedbow: Move ComposerExecutableValidator into Package Manager

automatic_updates.services.yml

package_manager/package_manager.services.yml

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

...e_manager/src/EventSubscriber/StageValidatorInterface.php

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

package_manager/tests/src/Traits/ValidationTestTrait.php

package_manager/tests/src/Unit/ValidationResultTest.php

src/Validator/PackageManagerReadinessCheck.php

...l/ReadinessValidation/ComposerExecutableValidatorTest.php

tests/src/Traits/ValidationTestTrait.php