Avoid multiple parallel calls to HostLoadImportedModule with the same (referrer, specifier) pair
Simplify requirements of HostLoadImportedModule
This is not necessary for this refactor, but it will be needed by Layer 0 of the compartments proposal if it wants to avoid calling importHook twice for await Promise.all([import("x"), import("x")]).
This is not necessary for this refactor, but it will be needed by Layer 0 of the compartments proposal if it wants to avoid calling
importHooktwice forawait Promise.all([import("x"), import("x")]).