Split package into top-level Python library.
We need to propagate it just like any other package, otherwise
dependents cannot detect it during installation.
The __init__.py collide since it uses old-style namespaces so we get rid
of those. In the future we may need to extend the namespaces.
This was enough for my dependency opencensus-ext-azure.
Having it as a top-level Python is just common practice; there is no
good reason not to.
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