Change module resolution extensions (#5801)
Change module resolution extensions (#5801)

Same as https://github.com/zeit/ncc/pull/80
azukaru/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,BlancheXu/test,flybayer/next.js,BlancheXu/test,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,BlancheXu/test,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js