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
Starting from 5.0.0, all apps in apps dir should follow semver
strictly. Do not use the umbrella project's release tags.
The universal tags made sense when we have applications scattered
in different repos. Now since this is an umbrella project, it is
enough to have a single package level version number corelated with
git tag.
