npm global folder ~/.npm-packages triggers env.modulePath

I'm not sure if this is a feature or a bug but it came up in https://github.com/jspm/jspm-cli/issues/1338.

The issue is that when the global npm install folder is a local user path, this is setting `env.modulePath`, which is normally not set when running the global version.

Let me know if I'm missing something here but perhaps liftoff can determine if the module is resolving to one of the global folders in https://nodejs.org/api/modules.html#modules_loading_from_the_global_folders and not set `env.modulePath` in these cases?