Make WebHost internal to avoid type conflict

[JunTaoLuo]

The type lives in the Microsoft.AspNetCore.Hosting.Internal namespace. Now that we have the WebHost as a public type in Microsft.AspNetCore that contains the static APIs, we should rename the one in hosting to prevent potential type conflicts.

HostingWebHost, InternalWebHost?

Note that this should not happen often since it's unlikely that anyone should be using the M.A.Hosting.Internal namespace.

[JunTaoLuo]

Could also potentially make the WebHost internal instead of public but @davidfowl seems to prefer renaming it instead.