[Bug] PublicApiContractTest fails to output public API information for some assemblies

[filzrev]

Describe the bug
PublicApiContractTest.cs generate Api.verified.cs file that describe public API metadata.

But it seems some assembly's public APIs information are not exposed.

List of assemblies that is not outputted API metadata`

• Docfx.Build.RestApi
• Docfx.Build.TagLevelRestApi
• Docfx.Build.UniversalReference
• Docfx.DataContracts.RestApi
• Docfx.DataContracts.UniversalReference
• Docfx.DataContracts.OperationLevelRestApi

It seems assembly reference is removed when type is not actually used inside dlls.
So It's required to load these assemblies manually before calling GetReferencedAssemblies.

Expected behavior
All public API contracts are outputted

Additional context
Add any other context about the problem here.