The reproduction of issue #7421 involves a list of maps being passed to
a module, where one or more of the maps has a value which is computed
(for example, from another resource). There is a failure at the point of
use (via lookup interpolation) of the computed value of the form:
```
lookup: lookup failed to find 'elb' in:
${lookup(var.services[count.index], "elb")}
```
Where 'elb' is the key of the map.
