This PR intended to fix issue that reported at #9988.

When using Docfx.App 2.76.0 or later.
It need to explicitly add references to Roslyn packages.

Background
Roslyn 4.9.2 or later packages contains BuildHost related contentFiles.
And these files needs to be copied to output directory.

It works as expected when using ProjectReference.
But when using PackageReference and these files are not copied to output directory when packages are referenced as transitive.

To resolve this issue. It need to add direct package references manually.
NuGet/Home#6826