Adopt split repo: python-api-core

[vchudnov-g]

Migrate the split-repo https://github.com/googleapis/python-api-core to https://github.com/googleapis/google-cloud-python. To do the actual migration, we need to ensure we can clear any of the following tags in this issue that describe the state of the source repo: testing:system, samples:manual,workaround:owlbot.