Make ns.foo trigger evaluation also for non-existing properties

[nicolo-ribaudo]

This change solves a few problem:

1. There was currently no way to trigger evaluation of a deferred module with no exports (see No named exports edge case #19 by @guybedford). I was initially leaning towards just banning import defer * as ns, but the solution in this PR also has the benefit in (2).
2. When implementing import defer * in tool, it's not always easy to know the list of bindings exported by a module at runtime without evaluating it. Examples are Webpack and Babel, that get the list of exports from properties of an object populated by the module evaluation. It would be possible in some cases to hard-code the list of exports, but it introduces additional complexity and duplicated information in the generated bundle.

Symbols still do not trigger evaluation, because:
a. it's guaranteed that exports of a module have string names
b. module[Symbol.toStringTag] === "Module" can still be used to detect probable namespace objects without forcing their evaluation.

[guybedford]

I must admit I did prefer the pure model framing in #19 (comment) in making this an error. I understand if detecting the list is tricky in unbound tooling workflows, but I'm not completely sure I agree that justifies omitting the error.

[nicolo-ribaudo]

I will prepare a PR for that — I don't have a super strong preference between these possible behaviours

[Jack-Works]

but the ecosystem also uses module[__esModule] to interop

[nicolo-ribaudo]

Since it's a known string it can be easily worked around. For example, the webpack implementation doesn't trigger evaluation on __esModule.