fix(packages): run composer as a separate process and roll back failed installs

[elcreator]

package:installrequire ran Composer inside the artisan process. Composer then replaced the vendor files that process was loading classes from; a class loaded lazily halfway through the update was already gone, every remaining operation failed and the site was left with a broken vendor directory.

• Run Composer through ExecWithFallback as a separate process, found the same way the core updater finds it (shared RunsComposerShell trait). The in-process run stays only for hosts where no process can be started.
• On failure, or when the site no longer boots afterwards, restore custom/composer.json and composer.lock and reinstall from the old lock.
• Pass --minimal-changes where Composer supports it, so shared dependencies move only as far as the new package needs.
• package:installautoload rebuilds the autoloader instead of updating every package.
• Store "Install by file" recognises a Composer package archive, keeps it in a local artifact repository and queues a console install for it, which works offline when its dependencies are installed. An archive shaped like a Composer package with an unusable composer.json is refused instead of being copied into the web root.
• cli-install reports a failed composer update instead of passing over it.