cue: cmd/cue: unclear how to validate selector and qualified identifier errors

_Originally opened by @myitcv in https://github.com/cuelang/cue/issues/629_

What version of CUE are you using (cue version)?

$ cue version
cue version +40d9da31-dirty linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

Consider the following:

exec cue eval x.cue

-- cue.mod/module.cue --
module: "mod.com"
-- fn/fn.cue --
package fn
-- x.cue --
package x

import "mod.com/fn"

s: {}

a: y    // ERROR: reference "y" not found
b: s.y  // not an error
c: fn.y // not an error

gives the following:

a: reference "y" not found:
    ./x.cue:7:8

This error above is as expected.

~According to my reading of the spec, both s.y and fn.y evaluate to _|_. Hence, strictly speaking, the output above is correct.~

But the lack of errors for lines 8 and 9 are tricky as the author of this code. I have no way of knowing that I have referenced non-existent fields until evaluation time. ~My error has been masked by the use of a disjunction.~

Note: this repro is derived from a real-world example where this typo was made across tens of files.

What did you expect to see?

Unclear, because it’s conceivable that a different user might have intended the above behaviour, and so not expect to see errors for s.y or fn.y

cc @jlongtine

2022-05-03 edit: removed use of disjunctions, because this issue is primarily being referenced because of the lack of errors/UX issues that come with incomplete references. The disjunction case is an interesting extension of that same problem, but is one we necessarily need to solve second.