Is there a way to create a private resource implementation without publishing anything anywhere? I'm working on a private monorepo, and would like to have a custom resource implementation that does not make any sense to the external world. I'm willing to do the necessary work to build the necessary provider binary, but I don't know where to put it for Pulumi so that it does not go and try to fetch it from somewhere else.

Iirc the go provider template actually does this. In any case you can generate an sdk and use it as a dependendency with a path