SDK package is setup as a Regular Package when it should be a Namespace Package

[NathanielRN]

As a follow up to @aabmass 's comment on #769, the __init__.py file in the SDK package causes it to be interpreted as a Regular Package instead of a Namespace Package. See the two types of Python packages. Namespace Packages should not have an __init__.py file.

Namespace packages are meant to contribute sub-packages (e.g. sdk.metrics, sdk.traces, sdk.resources) to some parent package (i.e. opentelemtry.sdk).

Having this __init__.py file makes it so that we cannot extend the opentelmetry.sdk package to have additional sub-packges (e.g. opentelemetry.sdk.extension).

The solution should be the same as in #769 , where we change the file to be called __init__.pyi instead. This should allow mypy to know this is a namespace package without preventing future sub-packages from being added.