Missing references with interface implementation

**Operation System**: (`Windows` or `Linux` or `MacOS`)

Windows

**DocFX Version Used**:

2.47

**Template used**: (`default` or `statictoc` or contain custom template)

Irrelevant

**Steps to Reproduce**:

1. Unzip the attached sample
2. Run builddocs.sh (or run the steps manually)

**Expected Behavior**:

The documentation for Library1.Foo should be able to refer to the documentation for Library2.IFoo, given the xref map reference in Library1/docfx.json

**Actual Behavior**:

The xref map appears to be loaded, but the external reference can't be found:

```text
[19-11-06 09:37:12.506]Info:[BuildCore.Build Document.LinkPhaseHandlerWithIncremental.UpdateContext]0 external references found in 2 xref maps.
```

(This is then visible in the generated docs - I'd expect a link, but it's just shown as Library2.IFoo in the docs.)

This may well have the same cause as #4792.

Docfx becomes *significantly* less useful for multi-library documentation when cross references can't be resolved. I may try to work out how to debug docfx myself to investigate further, given the lack of attention to the possibly-related bug.
[MissingReferences2.zip](https://github.com/dotnet/docfx/files/3813755/MissingReferences2.zip)