Remove support for chained IServiceProvider fallback from ServiceProvider

- Replaced with explicit IServiceManifest/IServiceCollection.Import
- Still need to verify all up: hosting/routing/mvc/identity repo changes

cc @davidfowl @lodejard @halter73 @divega 

Please review the tests and container code removed carefully, they all appeared to be testing/related to fallback, but I'm not 100% sure
The param name is misleading, these are extension methods on IEnumerable, not IServiceCollection :(  That was the first thing I tried...