JSON serialization: namespaces for cirq_type

[mpharrigan]

Right now in cirq and all modules in the main cirq repo, the cirq_type field used in json serialization is the unqualified object name. This has not caused any naming collisions yet.

I had always imagined that different packages would have a namespaced cirq_type. For example: cirq_google.PhysicalZTag or cirq.google.PhysicalZTag instead of PhysicalZTag.

In fact, cirq.obj_to_dict_helper has an optional namespace argument. I tried to use this for a new cirq_google object, but the testing procedure uses the unqualified object name to assert that a data file should exist.

While playing around with #4375 I think it would be more robust to decouple the "object generation" from any semantic meaning. Instead, we generate all the example objects and keep track of the types we've successfully round-tripped. Afterwards we can check if there are public objects that weren't round-tripped.

[mpharrigan]

#4527 adds a hook into the current testing infrastructure to make it possible to make namespaced objects pass the tests, although it requires manually adding a mapping from unqualified class name to cirq_type, which doesn't solve the disambiguation problem.