Due to the wording in [1] the broader python tooling ecosystem
sometimes expects an exact match to the single line pkgutil extend_path
usage. This change replaces current pkgutil style namespace package
implementation with the one line implementation described in [1].

This change also causes an import order change as `poetry.core` is
correctly detected as a third-party package.

[1] https://packaging.python.org/guides/packaging-namespace-packages/#pkgutil-style-namespace-packages