Bug 1787926 - Don't add async dependency on modules that have finished evaluating. r=yulia, a=RyanVM

The problem is that we're waiting for an import to finish evaluation after it
already has.

I think this a problem with the spec, related to the change from
AsyncEvaluating to AsyncEvaluation here:

https://github.com/tc39/proposal-top-level-await/commit/c2375dbd6ecb0d5e9a415f6a812fc0974a2935b7#diff-181371b08d71216599b0acccbaabd03c306da6de142ea6275c2135810999805aL588-R589

Previously we wouldn't add an async dependency when the required module's
AsyncEvaluating field was false, which was the case after evaluation had
finished. Now this is never set to false so we always add the dependency here
and there's nothing to trigger evaluation of the importing module.

Differential Revision: https://phabricator.services.mozilla.com/D155968