I’d start here:
Go programs are organized into packages. A package is a collection of source files in the same directory that are compiled together. Functions, types, variables, and constants defined in one source file are visible to all other source files within the same package.
If those files are in the same folder as main.go, they should be in the same package (main) and instead of json.respondWithJSON you would just use respondWithJSON (since they’re in the same package). If you want to make a different package, put your files in a subfolder (for example /json) but I would also caution against creating a package called json because that conflicts with the stdlib.