emqx_moduels for enterprise is refactored as a lib/plugin
in order to avoid merge conflicts, we make sure they can
co-exist in the same branch, because lib-opensource is compiled
in opensource project and lib-enterprise (to be added) is
compiled in enterprise project
And move emqx app back to root level
After realising the challenges of developing plugins in standalone
rebar projects. it is perhaps more smooth to keep using emqx as
a rebar dependency
Zaiming Shi