A while ago I updated case #1450 with a note that routes are only
loaded from engines, not from vanilla plugins.
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.
f2ee056 Added view path support for engines [DHH]
3cc9d1c Let all plugins not just engines have a config/routes.rb file