Force generated assemblies to reference jsii-only dependencies.

[mpiroc]

When a NuGet package is restored, all of its listed dependencies are restored (whether or not they are actually used). But when a .NET assembly is built, if it does not reference any .NET types from the dependency, the compiled .NET assembly will not reference the dependency .NET assembly.

At runtime, jsii-dotnet-runtime loads JSII dependencies by looking through the .NET assembly's references. If the dependency does not appear in references, it won't be found.

This breaks the following scenario:

• A JSII assembly my-package depends on my-dependency.
• In the TypeScript source, my-package requires symbols from my-dependency. Therefore the NuGet package MyPackage lists MyDependency as a dependency.
• However, the generated .NET source for MyPackage does not reference any symbols from MyDependency.

At runtime, the first time a type from MyPackage is used, jsii-dotnet-runtime will search for MyPackage's dependencies, and ask jsii-runtime to load each of them (i.e. my-dependency) before attempting to load my-package. But due to the missing assembly reference, it won't be able to find my-dependency. So when the javascript code for my-package attempts to require('my-dependency'), it will fail.

To work around this issue, for each package, we generate a type MyNamespace.Internal.DependencyResolution.Anchor that references the Anchor from each of its dependencies. This way all listed dependencies will show up in the .NET assembly's references, avoiding this issue.

[costleya]

_ private (be obvious)

[mpiroc]

I'm find with adding explicit access modifiers where they are currently implied, but currently the codebase omits all redundant access modifiers. If we change this, we should change it across the whole codebase.

Created #217 to track this.