Add an API for comparing the (current) route name that takes into account deprecated routes

Started a MR with a proposal for the new method. Feedback welcome :)

Appears small feedback has been provided by @acbramley.

Wonderif there is a current instance in core that could use this and be converted to show it's worth?

Addressed all the feedback so far, but there's one thing left.. the main reason for this API: checking whether the passed-in route name is deprecated.

The problem is that the only way to do that currently is \Drupal\Core\Routing\RouteProvider::getRouteAliases(), which would mean an uncached db query for every call to the new RouteMatch::isRouteName() method. I think we need a way to retrieve and cache (persistent and static) all aliased/deprecated routes somewhere in the route provider. Setting back to NW to get feedback on that.

I'm a bit confused by #5 but I might have it backwards.

RouteProvider::getRouteByName() has this:

    $result = reset($routes);
    if ($result instanceof Alias) {
      $alias = $result->getId();
      if ($result->isDeprecated()) {
        $deprecation = $result->getDeprecation($name);
        @trigger_error($deprecation['message'], E_USER_DEPRECATED);
      }
      return $this->getRouteByName($alias);
    }

So... in isRouteName(), if it matches the current rout name, we can assume it's not a deprecated alias because no-one should ever actually land on one and just return TRUE.

If it doesn't match, we can call RouteProvider::getRouteByName() on each of the passed in routes, this will trigger the deprecation we need.

Route preloading should mean this isn't an additional database query. There is some rationalisation to do in 🐛 RoutePreloader loads a lot of routes Active but it's just another thing to take into account on that issue.