This PR changes the distribution name from django-google-spanner to google-cloud-spanner-django to match the other googleapis projects. Thankfully this project hasn't been released on PyPI yet, so we don't have to worry about deprecating the old name.

It also changes the package namespace. Now instead of a single top-level django_spanner we've got two new namespaces under the shared google.cloud: google.cloud.spanner_django and google.cloud.spanner_dbapi. This is (1) and (2) in the description, I just wanted to make it explicit that we're using namespace packages here and this isn't just a directory move.

Both great changes, and both following the conventions of the other python repos.

After talking to @skuruppu it sounds like we should keep the django-google-spanner distribution name:

• The google-cloud- name usually denotes a GCP service client, and this isn't exactly that.
• Most django apps are named django-. This is only convention, but a pretty strong one.
• Other google-owned django apps (like https://pypi.org/project/django-cloud-deploy/) follow this convention too.
• And so do other third party DB django apps.

@mf2199, this PR looks not very actual - most part is already merged. Should we close it?

Closing as this PR seems to be old, and its changes were merged with other smaller PRs