Fix creating fsm visualizer with lazy routes

Previously, the visualizer html could be created very simply with

```
$ rails runner "Rails.application.routes.router.visualizer" > viz.html
```

This doesn't work with `LazyRouteSet` because it doesn't trigger route
loading when calling `router` (so the visualizer ends up displaying no
routes). This could be fixed by added `router` to `LazyRouteSet`, but
since `.router.visualizer` is undocumented I'd like to propose moving it
to `Routes` instead. `LazyRouteSet#routes` already triggers loading
routes, and `Router#visualizer` ended up delegating all of its logic to
`Routes` anyways.

The command changes slightly, but remains as simple as before

```
$ rails runner "Rails.application.routes.routes.visualizer" > viz.html
```

This commit also includes two small changes along with the move.

The first is that `partitioned_routes` is replaced with
`Routes#anchored_routes`. This is an improvement because
`partitioned_routes` previously relied on `requirements_anchored?` which
can't be called after `eager_load` (it uses the path's `Journey::Ast`,
which is cleared on `eager_load`). `anchored_routes` get filtered with
the same conditions as `partitioned_routes` except the filtering is done
as `Route`s are added to `Routes`.

The other change is removing `return [] unless ast` from
`Router#find_routes`. This is safe because `Routes#ast` already always
returns a `Journey::Node` (and it enables removing `Router#ast` because
`#find_routes` and `Router#visualizer` were the only two users of the
method).

Author: skipkayhil

actionpack/lib/action_dispatch/journey/router.rb

@@ -98,24 +98,7 @@ def recognize(req, &block)
         end
       end
 
-      def visualizer
-        tt     = GTG::Builder.new(ast).transition_table
-        groups = partitioned_routes.first.map(&:ast).group_by(&:to_s)
-        asts   = groups.values.map(&:first)
-        tt.visualizer(asts)
-      end
-
       private
-        def partitioned_routes
-          routes.partition { |r|
-            r.path.anchored && r.path.requirements_anchored?
-          }
-        end
-
-        def ast
-          routes.ast
-        end
-
         def simulator
           routes.simulator
         end
@@ -125,7 +108,6 @@ def custom_routes
         end
 
         def filter_routes(path)
-          return [] unless ast
           simulator.memos(path) { [] }
         end
 

actionpack/lib/action_dispatch/journey/routes.rb

@@ -72,6 +72,13 @@ def add_route(name, mapping)
         route
       end
 
+      def visualizer
+        tt     = GTG::Builder.new(ast).transition_table
+        groups = anchored_routes.map(&:ast).group_by(&:to_s)
+        asts   = groups.values.map(&:first)
+        tt.visualizer(asts)
+      end
+
       private
         def clear_cache!
           @ast                = nil