debian/package: warn on incorrect package name w/ Passenger

According to Passenger docs:

"You should install nginx-extras even if you have already installed an Nginx package from the
official Debian/Ubuntu repository. This is because the Nginx binary that our packages supply
is compiled with the Passenger module."

manifests/package/debian.pp

@@ -61,6 +61,10 @@
           ensure  => 'present',
           require => Exec['apt_update'],
         }
+
+        if $package_name != 'nginx-extras' {
+          warning('You must set $package_name to "nginx-extras" to enable Passenger')
+        }
       }
       default: {
         fail("\$package_source must be 'nginx-stable', 'nginx-mainline' or 'passenger'. It was set to '${package_source}'")