Non-normative layering fix for #59

[guybedford]

This addresses the implementation feedback in #59 that expecting an internal slot on objects that might not be defined in ECMA-262 poses implementation difficulties and that a new host hook layering would be the preferable approach to the spec boundaries.

This change satisfies the branding requirement, while resolving the implementation feedback.

The new host hook provided to implement this new check is HostGetModuleSourceName which takes a module source object and returns its string value, throwing if it is not a module source object.

The strong branding of toStringTag is then retained by calling this hook.

[nicolo-ribaudo]

I think you should still present this, as even if not technically normative some delegates prefer layering changes to go throu plenary.

[nicolo-ribaudo]

@guybedford I was thinking about what the host implementation of this hook would look like, and it introduces much more complexity than the current spec because it forces us to answer the question of "given a user-created WebAssembly.Module not coming from the module loader, what is the corresponding module record?". While answering this question might be possible (even though we probably don't agree yet on what the answer should be, not even in the modules harmony group), it requires discussing with multiple parties.

It might be better to keep this 1:1 with the current spec and instead have a HostGetModuleSourceClassName that just returns a string. We can keep the discussion of how to build a module record from an arbitrary source in the other proposal, that will need to do it.

[guybedford]

Thanks @nicolo-ribaudo for the excellent point here. Yes let's match the existing behaviour for now and tackle the larger questions for ESM source phase layering in due course.

[lucacasonato]

I think this wording is clearer.