Consider `source` phase in `GatherAsynchronousTransitiveDependencies`

[caiolima]

This is not a real problem in the spec, but just a reminder when merging this with source phase imports we need to consider if a request.[[Phase]] == source on GatherAsynchronousTransitiveDependencies. IIUC, the expected behavior here is that we don't keep going through the recursion for this request, since the idea with import source is to not traverse the full dependence graph yet.