Dynamic module loading compatible with function mode

Will allow an app or library compiled into a single file to dynamically load external modules.

- [x] Separate `@nodepack/module` package
- [ ] Force this package to be external in function mode

Projects needing this will have to install `@nodepack/module` as dependency and it will also a dependency in the built output. (It has only [one dependency](https://github.com/Akryum/nodepack/blob/master/packages/%40nodepack/module/package.json#L24), no transitive dependencies.)