Update cirq_google to import from cirq directly

[maffoo]

Now that cirq-google is a separate package from cirq-core, we can import cirq like a separate library. We use the top-level cirq namespace wherever possible instead of importing subpackages, and also remove some of the if TYPE_CHECKING guards and unquote cirq type annotations.

[balopat]

This is great, I didn't think of this consequence of the extraction. It looks much cleaner this way - like using the "public API" of the cirq library instead of depending on the package structure. Thanks!