When using name_prefix on an import, RouteCollection::addNamePrefix() unconditionally prefixes the target of all aliases.
If an alias points to an external route (a route not present in the imported collection), the link breaks because the external route name is not actually changed.

This PR ensures we only prefix the alias target if that target exists within the collection being modified.

# config/routes.yaml
admin_section:
    resource: routes/admin.yaml
    prefix: /admin
    name_prefix: admin_

Inside routes/admin.yaml, I have an alias pointing to a global route defined elsewhere (app_logout):

# config/routes/admin.yaml
logout:
    alias: app_logout

Before:
php bin/console cache:clear fails because the dumper tries to find the prefixed target (admin_app_logout) which does not exist:

In CompiledUrlGeneratorDumper.php line 72:
Target route "admin_app_logout" for alias "admin_logout" does not exist.

After:
The alias correctly points to the original app_logout route and the cache warms up successfully:

[OK] Cache for the "dev" environment (debug=true) was successfully cleared.