build should more smart

[fishbar]

when both entry files A.js & B.js require [c.js, d.js, e.js], now cube will treated [c.js, d.js, e.js] as root files too， and will not merge merge them. so it will slow down loading speed.

since these file no need to be root files，so they can be put into some group and merged togather，it will helpful to reduce root files and speed up page loading.