Emit `__esModule` for every ES6 module emitted.

[mhegazy]

In anticipation for change by the Node CTC to adopt ES6 modules, transpiled modules should all have the flag set. this gives an unambiguous way to know if this is a trsanspiled module of a plain old CJS module.

[yuit]

Export __esModule for all module except when there is export =
Update: such export should be before calling of require to facilitate circular dependency