Support require of global libraries

Currently there is no disciplined way to interact with global libraries. Access is always via `js/foo`. Instead if we provide a non-clashing convention for talking about global libraries, interaction becomes more disciplined. Benefits include the ability to validate via externs when available:

For example:

But this will also work for many other contexts, say a JavaScriptCore application embedded in iOS:

This could also work for Rhino / Nashorn / Graaljs, for example: