Modular cirq: cirq.google is extracted

[balopat]

This PR extracts cirq.google into cirq_google.

This is a huge change that I don't really see how could be broken up to smaller pieces. It will probably stick around for a couple of weeks until all the downstream system's testing is done.

Major features:

• user code using cirq.google is going to get a DeprecationWarning, but otherwise will still work
• cirq will become a "metapackage"
  • pip install cirq will install cirq, cirq-core and cirq-google
  • pip install cirq-google will install cirq-core and cirq-google
• all scripts, CI config, notebooks and release scripts have been reconfigured

Testable versions:

• https://test.pypi.org/project/cirq-google/0.11.0.dev20210323153007/
• https://test.pypi.org/project/cirq-core/0.11.0.dev20210323153007/
• https://test.pypi.org/project/cirq/0.11.0.dev20210323153007/
• pip install cirq==0.11.0.dev20210323153007 --pre --index-url=https://test.pypi.org/simple --extra-index-url=https://pypi.org/simple should install the cirq-google, cirq-core packages and the cirq metapackage

The plan: get consensus and sign-off from all major stakeholders and impacted downstream projects (cirq, qsim, quantum OS, TFQ, QCS) and then merge. I do have the changes somewhat scripted that makes it easier to keep the branch up to date with master - there is probably not going to be a need for a code freeze as such.

There are several commits that allow for easier review:

• in-place changes
• new files
• cirq.google -> cirq_google rename
• final move of the modules

Note that this includes currently #3948 which adds an extra commit at the beginning.

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[95-martin-orion]

Bundle of setup.py nits.

[95-martin-orion]

LGTM from me. As double-approval has been requested on this PR, I'm leaving the final "Approve" flag-flip to Doug.