Consider lazy loader for heavier imports

[dabacon]

Description of the issue

There are a couple of packages that Cirq uses that have slow import times, but are not heavily used. These include a lot of subpackages of scipy, like scipy.optimize and scipy.stats. Currently we lazily import these before their use in order to avoid these being imported in the global init of cirq. See #4643 for example.

However @MichaelBroughton suggested that we do this the way that TensorFlow does this, using the following class
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/util/lazy_loader.py#L23

We should consider doing this, and if so we do we should also test the LazyLoader class.

[MichaelBroughton]

Some more discussion in #4643

[dabacon]

Implemented in #4653