Using MapFallbackToPage with a page called Index.cshtml leads to an ambiguious match

**Repro**

1. Create a new Server-Side Blazor app.
2. Rename `Pages/_Host.cshtml` -> `Pages/Index.cshtml`.
3. Navigate to `/Index` in the app.

This results in an ambiguous match from routing because a page named `Index` produces two endpoints. This isn't ideal because it can be a useful pattern to place a page in the app route and then use fallback routing.