WebApplicationBuilder adds default services twice

[davidfowl]

After my change got merged Swashbuckle broke when using the WebApplicationBuilder and it's because we call ConfigureWebHostDefaults twice:

1. aspnetcore/src/DefaultBuilder/src/WebApplicationBuilder.cs

   Line 52 in 348b810

   _deferredHostBuilder.ConfigureWebHostDefaults(configure: _ => { });

2. aspnetcore/src/DefaultBuilder/src/WebApplicationBuilder.cs

   Line 111 in 348b810

   _hostBuilder.ConfigureWebHostDefaults(ConfigureWebHost);

Assuming it was fine to do so but it isn't when using API explorer because we end up adding the same provider twice which makes swashbuckle (and I assume other code) unhappy. Swagger/OpenAPI is currently broken because of this.

cc @halter73 @LadyNaggaga @bradygaster

[halter73]

Yeah. I knew this might be bad. That's why I left this comment saying "We call ConfigureWebHostDefaults AGAIN...". 😆

aspnetcore/src/DefaultBuilder/src/WebApplicationBuilder.cs

Lines 108 to 111 in 348b810

	// We call ConfigureWebHostDefaults AGAIN because config might be added like "ForwardedHeaders_Enabled"
	// which can add even more services. If not for that, we probably call _hostBuilder.ConfigureWebHost(ConfigureWebHost)
	// instead in order to avoid duplicate service registration.
	_hostBuilder.ConfigureWebHostDefaults(ConfigureWebHost);

We'll have to consider how we deal with the "ForwardedHeaders_Enabled" scenario.

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