- #549, #592
- move feature implementations to `Microsoft.AspNetCore.Http.Features` and `...Authentication.Features`
- move `DefaultHttpContext`, `HttpContextAccessor`, `HttpContextFactory`, `FormCollection` and
`HeaderDictionary` to `Microsoft.AspNetCore.Http`
- move `FormFile` to `Microsoft.AspNetCore.Http.Internal`
- that and `Microsoft.AspNetCore.Http.Authentication.Internal` are the remaining `.Internal` namespaces
nits:
- remove a couple of parameterless constructors
- add / fill subfolders to align with new namespaces
- remove all use of (unnecessary) "T:..." `<see cref=""/>` values
6725d68559