compilation using project references fails if a tsconfig.json includes an external .d.ts.

### Expected Behaviour

successful compilation.

### Actual Behaviour

Error: Debug Failure. False expression.
    at Object.getOutputDeclarationFileName

### Steps to Reproduce the Problem

just clone the repository and run:
```
yarn install 
yarn build
```

### Location of a Minimal Repository that Demonstrates the Issue.
https://github.com/gvinaccia/ts-loader-issue