I thought it would be as easy as creating a new folder, copy-pasting source tree into it and referencing the lib from a source file. But the copied files do not show up under the new folder in Solution Explorer sidebar. Obviously there is more than that.
What do I need to do to make this work?
You need to click on the “Show All Files” in the Solution explorer toolbar, and then right click on the folders/files and select “Include in project”.