#1450, loading route files from plugins

Hi all,

A while ago I updated case #1450 with a note that routes are only
loaded from engines, not from vanilla plugins.

https://rails.lighthouseapp.com/projects/8994/tickets/1450-add-way-for-plugins-to-map-routes

But apparently, it's impossible to reopen a closed case, so,
understandably it never got looked at.

Here's what I posted, reshuffled:

Looks like f2ee056 regressed 3cc9d1c.

That is, plugins can't have routes anymore, but engines do.

routing_files was replaced by add_engine_routing_configurations, which
again uses #engines instead of #plugins.

Referenced commits:
f2ee056 Added view path support for engines [DHH]
3cc9d1c Let all plugins not just engines have a config/routes.rb file
http://github.com/rails/rails/commit/f2ee056873b84f8917e72d87181e1a9f5f653342
http://github.com/rails/rails/commit/3cc9d1c5ad1639283b43ee2b6099cb4f3b19bf23

But apparently, it's impossible to reopen a closed case, so,
understandably it never got looked at.

It's impossible for you to, but I can. I've reopened it.