I offen manage css/js plugin with composer which install it under vendor. I symlink those to the web/bundles directory, that's let composer update bundles as needed.
exemple:
1 - symlink once at all (use command fromweb/bundles/
ln -sf vendor/select2/select2/dist/ select2
2 - use asset where needed, in twig template :
{{ asset('bundles/select2/css/fileinput.css) }}
Regards.