refactor: rename loader to definitions

src/Binding/Loader/ArrayBindingLoader.php

src/Binding/Loader/FilesystemDefinitions.php

src/Extension/ConditionalExtension.php

src/Extension/ConfigExtension.php

src/Extension/LegacyExtension.php

tests/Binding/ArrayBindingLoaderTest.php

tests/Binding/CompositeBindingLoaderTest.php

tests/Binding/DirectoryBasedLoaderTest.php

tests/HookDriver/GenericDriverTest.php