evaluator: reference "strings" in label expression refers to field against which it would be matched

[rogpeppe]

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

v0.16.0

Does this issue reproduce with the latest stable release?

Yes (v0.16.0)

What did you do?

exec cue vet x.cue

-- x.cue --
import "strings"

[strings.MaxRunes(3)]: _

What did you expect to see?

A passing test. Unlike the similar error showing in #2013, this does not involve any kind of cycle.

What did you see instead?

> exec cue vet x.cue
[stderr]
reference "strings" in label expression refers to field against which it would be matched:
    ./x.cue:3:2
[exit status 1]