Several changes to lookup to improve how we handle marked values:
- If the entire collection is marked, preserve the marks on any result
(whether successful or fallback)
- If a returned value from the collection is marked, preserve the marks
from only that value, combined with any overall collection marks
- Retain marks on the fallback value when it is returned, combined with
any overall collection marks
- Include marks on the key in the result, as otherwise the result it
ends up selecting could imply what the sensitive value was
- Retain collection marks when returning an unknown value for a not
wholly-known collection
See also https://github.com/zclconf/go-cty/pull/98
dbe5272931