Circular module graphs instantiate too early

[domenic]

@jonco3 found this in https://bugzilla.mozilla.org/show_bug.cgi?id=1361988. Given the module graph

      test2
        ^
        |
  .-> test1 --.
  |           |
  '-- test3 <-'

(with test1 as the root)

it appears we instantiate subgraphs too early, causing assertion failures in HostResolveImportedModule.

We probably need to move all instantiation to the "top level", instead of doing it on subgraphs as we go.

I don't believe this is fixed by any of the in-flight PRs (#2595, #2604, #2625).

[domenic]

@jonco3 points out in IRC that instantiate was originally top-level, but that was changed for reasons discussed in #1545. Instead he's thinking maybe we can solve this by marking modules that are part of a cycle and not instantiating them eagarly, using the ancestor list.