[Bug] docfx should tolerate missing metadata references for project references

docfx seems to _really_ dislike when there is a project reference without a compiler reference assembly. It emits warnings like this and a failing exit code:

```
warning: [Warning] Found project reference without a matching metadata reference: E:\src\vs-mef\src\Microsoft.VisualStudio.Composition.Analyzers.CodeFixes\Microsoft.VisualStudio.Composition.Analyzers.CodeFixes.csproj
```

But there are _legit cases_ for having a reference to a project that does not offer a reference assembly. For example when I reference a project that produces a C# analyzer. I don't want to reference that in my own compilation, but referencing it as a project reference is required to get it to show up as a nuget package dependency.

Please offer a way to suppress this warning.

**Context (please complete the following information):**
- OS: Windows 11
- Docfx version: 2.78.5