Proposal: move ion and neutral into devices

[dabacon]

Currently ion and neutral, which are devices plus decomposition methods sit at the top level of cirq and get imported into the cirq.* namespace. Having these as top level modules is a bit odd since they are really about a particular device (and gates). I suggest that we move the files in these directories into the devices module (or maybe subdirectories of devices?)

I think they are generic enough that we can keep them at the cirq.* namespace, (aqt relies on them as does the coming pasqal).

The question then comes up about the device in aqt. But aqt is an api, so I think having api's in submodules cirq.aqt and cirq.google which also have devices is a good encapsulation.

[dabacon]

Alternative: create separate module which contains them (architectures)?

[balopat]

This is part of #3228. I believe that we should do this.

[MichaelBroughton]

xref #5130 , #5128

[dstrain115]

Discussion from cirq cync: Let's move neutral atoms into Pasqal, let's either move IonDevice into ionq (or delete it). Reason is that neither is used by more than one package. cirq-core will have a stronger deprecation policy so let's put them into vendor packages, so that, if needed, they can be a feature add rather than a later deletion.