Need a way to compile assemblies without dependencies on Unity or KSP

[JonnyOThan]

SCANsat has an assembly that needs to be included in its unity project, which means that it cannot depend on KSP's Assembly-CSharp. There may be a workaround here by providing a stub dll with the things that it would need (mostly KSPAssembly and KSPAssemblyDependency attributes).

KOS has an assembly that should not depend on Unity nor KSP - the core of the mod is actually a totally pure C# assembly so that it can be unit-tested outside of the game.

In both cases, the assemblies should be compiled by KSPBuildTools just like actual mod assemblies and copied to the binary output directory, etc.

To support these use cases, we should add a mechanism to disable the auto-referencing of the unity and KSP assemblies. Something like:

<PropertyGroup>
  <ReferenceUnityAssemblies>false</ReferenceUnityAssemblies>
  <ReferenceKSPAssemblies>false</ReferenceKSPAssemblies>
</PropertyGroup>