Remove `declare_namespace` calls in package

[jackwotherspoon]

Need to move off of using pkg_resources.declare_namespace() for package structure as it is deprecated.

DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google.cloud.sql')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages