Like the original poster, I created a blank plugin and modified the .build.cs file according to the directions in the UnrealBuildTool/ThirdPartyLibraries docs.
This instructs us to add a Type = ModuleType.External; statement in the build file’s constructor. With that statement, builds fail with the error posted previously “‘Plugin’ is not a C++ module (reference via default plugins → Plugin.uplugin)”
If I remove the Type assignment, I instead get an unresolved external error when trying to link against the dll for the plugin.
If others have a solution to this build configuration challenge, I’d love to hear it. I’m now looking at the example third party library plugin generated from the plugins panel and will try replicating its configuration (so there is both a dll-generating plugin and a third-party build within the plugin structure).