Move pubsub code into a subpackage

[dhermes]

This PR was created via: https://gist.github.com/dhermes/e239691aa584bd56a5352e34aad27cf3

export PROJECT_DIR="${HOME}/google-cloud-python"
export READMES_DIR="${HOME}/i-wrote-some-readmes-for-2357"
cd ${PROJECT_DIR}
git worktree add -b make-pubsub-subpackage ../hotfix official/master
python make_commits.py \
  --git-root "${PROJECT_DIR}/../hotfix" \
  --package pubsub \
  --package-name "Google Cloud Pub/Sub" \
  --readme "${READMES_DIR}/pubsub/README.rst" \
  --extra-requires $'    \'grpcio >= 1.0.0\',\n    \'google-gax >= 0.14.1, < 0.15dev\',\n    \'gapic-google-pubsub-v1 >= 0.9.0, < 0.10dev\',\n    \'grpc-google-pubsub-v1 >= 0.9.0, < 0.10dev\',\n'

NOTE: Since only pubsub and logging had to remove extra deps from GRPC_PACKAGES in setup.py, the 3rd commit is just written the old-fashioned way.

[tseaver]

LGTM

[dhermes]

Not going to merge this, going to merge #2420 and GitHub will magically close this PR.