External module support for ES6 module output

[theseanl]

Emitting ES6 modules is being worked in #723 (Disclaimer It doesn't preserve export statements in entry files, import and export statements are only used for inter-chunk references.) It currently works only with self-contained builds without external modules. More and more libraries are now being provided as ES6 modules, and it would be nice if we can make ES6 output modules reference external modules via import statements, not via global variables. It would be the best if it can be done in a way as easy as the "shim modules".