cmd/cue: provide a mechanism to download and cache all dependencies without updating cue.mod

[jpluscplusm]

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

I have a CI pipeline containing several steps that use cue.
My CUE module uses modules from a registry.

I would like to invoke a command near the start of the pipeline that fetches all my module's dependencies and leaves them in the module cache (so that only this one command needs to have access to auth tokens for the registry).
I would also like this command not to potentially update anything inside cue.mod.

Describe the solution you'd like

As per @mvdan's suggestion in Slack, I would like something akin to go mod download to exist.

Describe alternatives you've considered

• cue mod tidy && git check-that-cue.mod-has-not-been-updated
• cue mod tidy --check (but is this guaranteed to fetch every transitive dependency's full module contents?)

Additional context

n/a