Extern aliases not supported for go to definition and find references

## Version Used
VS Community Version 18.2.1
Roslyn 5.0.0-2.26054.2 @ ec501e4c6f2689c9c71e38ce72d892675965c392

## Steps to Reproduce

Assume the project `OtherProject.csproj` that contains the following namespace:
```cs
namespace OtherProject.Core;
```

Linked in `MainProject.csproj` like this:
```xml



```

In our `MainProject` we use `OtherProject` via `extern alias` like this:
```csharp
extern alias Core;

using Core::OtherProject.Core;
```

Place the cursor on any of the occurrences of the **extern alias** `Core` (not the namespace). Invoke either go to definition or find all references.

## Expected Result

Both GTD and FAR work.

## Actual Result

Neither GTD nor FAR work.

Reproduce the issue using OtherProject.csproj and MainProject.csproj with the shown ProjectReference alias and extern alias code. Start by testing Go to Definition and Find All References on the alias occurrence; done means both commands resolve and report references for Core.

Written by the indexing model from the issue text.