Prioro to this change, ekka lib dir was added by inspecting
the ekka lib version in persisted RELEASES file.
However, this file may not be the true version if a hot
upgrade failed in the middle.
In this fix, it reads the per-version emqx.rel file
content to load lib paths.
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