Importing ES6 modules

We're trying to embed Spidermonkey76 - will look move to 78 ESR at some point - into a software that isn't a web browser. We've been looking at support for ES6 features and aren't sure about modules.

Attempts to use local path identifiers with import commands (import MyFunc from "./whatever";) provoke an "import declarations may only appear at the top level of a module" message.

There was a similar post a couple of years ago. One response suggested the usage of JS::CompileModule (instead of JS::Compile<Script>). How do we know that we have a module a-priori, rather than a script? 

Is the solution then to use a module resolve hook? Would it be possible to find an example? The JS API user guide doesn't include much on this topic.

Thanks very much.