Single file exe + razor nuget breaks routing in asp.net core application

### Describe the bug
In any asp.net core project using the new .NET 5 single file exe publish command, routing is broken due to the use of Assembly.CodeBase. This impacts routing calls such as:

- `IEndpointRouteBuilder.MapControllerRoute`
- `IApplicationBuilder.UseEndPoints` and then calling either `MapRazorPages` or `MapControllers`

There are probably others.

### To Reproduce
See attached zip at bottom of thread. This appears to be a problem of adding some razor functionality to the service collection.

Run the published exe with a --urls parameter and then open your browser and attempt to hit the weather forecast end point.

### Exceptions (if any)
Use of Assembly.CodeBase throws not supported exception and application crashes.

### Further technical details
- ASP.NET Core version 5.0
- Does not reproduce in debugger. You must use dotnet publish command and turn on the single file option with `/p:PublishSingleFile=$publishSingleFile`

For servers obviously this can be worked around, just turn off the single file option. For client software that uses a web browser UI however this is unfortunate as the single file exe vastly reduces the number of files the client has to mess with to get the application deployed.