Install runtime as public assembly
The runtime dll needs to be placed in one of the public assemblies folders, registered in the registry user keys. See https://support.microsoft.com/en-us/help/306149/how-to-display-an-assembly-in-the-add-reference-dialog-box The Initialize method of the VSIX package can then check to make sure the dll is added as a reference to the Projects in the loaded solution.
A better solution would be to use NuGet packages for this (added as enhancement in #1) but this solution would be quicker to implement.