cue/vet: Display obvious errors without -c flag

[asdine]

Is your feature request related to a problem? Please describe.
I'm trying to use cue vet as a linter in a VS Code extension I'm building. Without the -c flag, running cue vet doesn't return anything other than some instances are incomplete; use the -c flag to show errors or suppress this message, even when there are other errors in the file or package (such as unknown identifiers, not syntax issues).
Enabling the "-c" flag returns lots of "incomplete value" errors for all the imported packages which "hide" the actual user errors.

Describe the solution you'd like
I believe the current behavior is useful when we're about to export our configuration but not really useful during development.
Ideally, cue vet without a flag should return all "obvious" errors found in the current package ("obvious" then needs to be defined) and not consider non-concrete issues.

Describe alternatives you've considered
I considered using -c and ignore all "incomplete value" messages but I'd rather rely on the cue vet's default behavior.

Additional context
This behavior is observable when having one or more third party packages in the cue.mod directory and importing them in one or more cue files at the root.

[jpluscplusm]

This is related to #2120. The problem might be helped by using the -c flag's negative form (-c=false), depending on the user's need.

This negative form isn't well documented just yet ... but we're addressing that at the moment.