External interfaces included in IDE symbol space

#### Overview

Interfaces defined in `.d.ts` files (even if they are not part of the Visual Studio project) are nonetheless included in the IDE symbol space; i.e. they appear in the Navigate To search results.
#### Repro

Add the NuGet package `underscore.TypeScript.DefinitelyTyped` to a project. Ensure that `underscore.d.ts` is not included in the project; however, do put a reference to it in a TypeScript file in the project, e.g.

```
///
```

In Visual Studio, press Ctrl + , and type `reduce`. Notice you get lots of listings for methods defined in the underscore interface.
#### Commentary

The repro steps are analogous to adding a reference to `System.Web.Mvc`, hitting Ctrl + ,, typing `file`, and seeing the `File` methods for `System.Web.Mvc.Controller`, plus who knows what else. Of course, for assembly references this doesn't happen; it would be noise, polluting the IDE symbol space with stuff you rarely want to try to navigate to. Libraries have lots of symbols, often more than our own apps. Those symbols get in the way of navigating quickly within our own app. That's why there's a switch to turn off external items. The same situation applies to JavaScript libraries with TypeScript annotations - not at the same scale as the .NET Framework, but still at enough scale to be a problem.

The bug is that for TypeScript, interfaces and their members in `.d.ts` files always show up. Ideally, they shouldn't show up regardless of whether the `.d.ts` file is referenced with an XML directive or by including the file in the project.

The main use case where it would be helpful to include the identifiers in the IDE symbol space is when developers are actually writing `.d.ts` files. To serve that need, it would be reasonable to include the symbols if the "Search within external items" option is enabled.

Reproduce the issue with underscore.d.ts referenced by an XML directive, then investigate how Navigate To indexes interfaces and members from external .d.ts files. The fix should keep those identifiers out of the IDE symbol space by default while allowing them when “Search within external items” is enabled.

Written by the indexing model from the issue text.