Define rules for `extend` declarations (`extend require`, `extend impl
as`, `extend base`, `extend adapt`) that say the target scope they name
must be complete at the point of the declaration. Define completeness
for a facet type to include all interfaces and named constraints that
provide unqualified name lookup through the facet type.
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Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
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