Feature Request: Namespaced autocomplete

Right now you either get all types from unimported namespaces, or the immediate children if you type out the namespace.

With "namespaced autocomplete", if we have:
```cs
namespace Foo {
namespace N1 { public class SomeClass1; }
namespace N2 { public class SomeClass2; }
}
namespace Bar {
namespace N3 { public class SomeClass3; }
namespace N4 { public class SomeClass4; }
}
```
then dotting off either namespace would only list types nested in that namespace.
```cs
Foo. // list only SomeClass1, SomeClass2
```
This is still considering unimported namespaces but scoped only to the one that is spelled out.