feat!: migrate to v2.0.0

[larkee]

This PR is a preview of the 2.0 release.

TODO list:

• Fix docs.

• Describe changes in UPGRADING guide.

BREAKING CHANGES

• list_instances, list_databases, list_instance_configs, and list_backups will now return protos rather than the handwritten wrapper

[busunkim96]

Thank you for working on this!

• I noticed there doesn't seem to be a samples presubmit. I opened 335755839 to add those kokoro configs internally.

  Handwritten files have been moved from google/cloud/spanner_v1 to google/cloud/spanner

  I believe the other handwritten/generated mixes keep the generated surface in the versioned directory. What is the
  motivation for the change?
  https://github.com/googleapis/python-firestore/tree/master/google/cloud/firestore_v1
  https://github.com/googleapis/python-pubsub/tree/v2.1.0/google/cloud/pubsub_v1

• I'm not familiar with the Spanner client, are there particular files we should direct our attention to?

[larkee]

• I noticed there doesn't seem to be a samples presubmit. I opened 335755839 to add those kokoro configs internally.

Thanks! I wasn't sure what was missing.

Handwritten files have been moved from google/cloud/spanner_v1 to google/cloud/spanner

I believe the other handwritten/generated mixes keep the generated surface in the versioned directory. What is the
motivation for the change?
https://github.com/googleapis/python-firestore/tree/master/google/cloud/firestore_v1
https://github.com/googleapis/python-pubsub/tree/v2.1.0/google/cloud/pubsub_v1

The motivation was two fold. It separates the handwritten surfaces which makes it clearer to contributes what is written and what is generated. It also prevents having to customize the generated google/cloud/spanner_v1/__init__.py. However given that this file shouldn't change too much, it's probably reasonable to exclude it from generation and manually update it when required.

I'm happy to revert these files back to their original folder if you think that would be best.

• I'm not familiar with the Spanner client, are there particular files we should direct our attention to?

Most of the handwritten surfaces are used directly by users. The major ones are:

• client.py
• instance.py
• database.py
• backup.py
• `batch.py
• snapshot.py
• transaction.py

However, the files that changed the most during migration are:

• _helpers.py
• snapshot.py
• streamed.py
• transaction.py

[busunkim96]

I would prefer the handwritten code live in the versioned directory for consistency with the other libraries. It looks like firestore excludes __init__.py in synth.pyhttps://github.com/googleapis/python-firestore/blob/c3acd4a04745c93edb2f61bf9be6fa33f439f4b0/synth.py#L41

Fully generated libraries use the unversioned path google/cloud/{api}/ for the default alias (equivalent of google/cloud/spanner.py) https://github.com/googleapis/python-speech/blob/master/google/cloud/speech/__init__.py so there's also a small chance for confusion there.

[skuruppu]

I only reviewed UPGRADING.md so my LGTM is for that file.

[wyardley]

@larkee: In the end, the "breaking change" that shows up in the changelog was
migrate to v2.0.0 (#147) vs

list_instances, list_databases, list_instance_configs, and list_backups will now return protos rather than the handwritten wrapper

which would have been much more useful

[larkee]

@wyardley My apologies! I'll do my best to label changes better in future 👍 I'll update the changelog directly to show the more useful message