pkg/list: list.Flatten fails on non-final value

[rogpeppe]

This cue code:

import "list"
Items :: [... string] | *[]
x: list.FlattenN([Items], -1)

produces:

{
    "x": [
        []
    ]
}

not the expected:

{
    "x": []
}

It seems that the flatten code isn't treating Items as a list.

[cueckoo]

This issue has been migrated to cue-lang/cue#268.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.