Composer not Found

If Composer can't be found in the PATH of the web server user, then it won't be detectable.

You could probably solve this by running the following Drush command: drush cset package_manager.settings executables.composer /usr/lib64/plesk-9.0/composer.phar.

Or, if you don't have Drush, add this to settings.php:

$config['package_manager.settings']['executables']['composer'] = '/usr/lib64/plesk-9.0/composer.phar';

Closing this; the solution I offered in #2 should work, to the best of my knowledge, and I didn't hear back in this issue.