modules: cue.mod/{pkg,gen,usr} "v2"

[myitcv]

In v0.8.0-alpha.1 the first version of CUE to include experimental support for modules, we included rudimentary support for using existing cue.mod/{pkg,gen,usr} directories where they exist in the main module. Covered in more detail in https://cuelang.org/cl/1176005.

This issue is a placeholder for "v2" - i.e. whatever comes next.

In this space we are exploring:

• Published versions of generated modules, e.g. pre-published versions of the k8s modules which obviate the need for running generation locally.
• How to support users/projects/companies who want to generate and publish CUE from existing Go (and indeed other) code.
• ...

Note this indirectly covers future plans/work around cue get go. With a special mention therefore of #646.