Issue #2608174 by webflo, hussainweb: Update composer-merge-plugin to latest version

composer.json

composer.lock

vendor/composer/installed.json

vendor/wikimedia/composer-merge-plugin/.arcconfig

vendor/wikimedia/composer-merge-plugin/.arclint

vendor/wikimedia/composer-merge-plugin/README.md

vendor/wikimedia/composer-merge-plugin/composer.json

vendor/wikimedia/composer-merge-plugin/src/Logger.php

...ikimedia/composer-merge-plugin/src/Merge/ExtraPackage.php

.../composer-merge-plugin/src/Merge/MissingFileException.php

...wikimedia/composer-merge-plugin/src/Merge/PluginState.php

...imedia/composer-merge-plugin/src/Merge/StabilityFlags.php

vendor/wikimedia/composer-merge-plugin/src/MergePlugin.php