cmd/cue: debug logging for modules

[rogpeppe]

Is your feature request related to a problem? Please describe.

When debugging registry-related issues, it's useful to be able to see what's going on under the hood.

Describe the solution you'd like

All commands that talk to the registry should be able to have logging enabled, probably by using the -v flag.

Additional context

To be decided: do we want to enable multiple levels of debug info (for example, higher level operations, OCI level operations, low level HTTP requests, auth requests) ?

[myitcv]

Perhaps following Go here it should be -x but I hold that lightly.