[policy, no patch] Decide how to handle routes for MENU_DEFAULT_LOCAL_TASK

Part of the larger #1889790: [Meta] Allow modules to register links for menus, breadcrumbs and tabs (if not with hook_menu), yet it deserves to be major because it is stalling ALL conversions with default tasks.

Both of these routes do the same thing:
admin/structure/types/manage/article
admin/structure/types/manage/article/edit
If you click the "Edit" tab that appears, it results in the first (short) route.

No where in the UI is there ever a link to a default tab. Only in tests and navigating by URL are those visited.

In my opinion, the important part is the visual tab, not the existence of a /edit URL I can visit.

Proposal

As hook_menu() is converted to routes, provide no route for them. Leave the definition in hook_menu() with no route_name, which will leave the tab and the UI unchanged.
Change all tests to not directly visit MENU_DEFAULT_LOCAL_TASK URLs.