Normative: **Non-injective** ModuleSource identity

[guybedford]

#58 formalizes the module source identity requirement as presented at the last meeting, based on the host invariant formalization in #59.

This is a follow-on PR that removes the injective behaviour of module sources.

This was an early design decision for module source transfer, that if we had a module source for a module at /foo.js, passing that module source into a worker, and importing its source object, would allow it to be imported by URL afterwards to get the same instance:

const fooSource = receiveSourceFromMainThread();
const instance = await import(fooSource);
await import('/foo.js') === instance // TRUE

The way this works is that importing the source object performs an injection into the module registry of the worker, provided no URL was previously defined at that key.

The reason this was designed from the start was to ensure cycles, for example if /foo.js imports /bar.js and /bar.js then imports back to /foo.js. If the URL was not present in the registry (only the source object), then we would get two instances for foo - one for the foo source, and one for the /foo.js URL fetched from the worker separately to the one transferred and serialized.

In recent conversations and at the last module harmony meeting it was brought up that we should make sure we have talked through this constraint one last time.

The injective behaviour is racy - if the URL already existed, it would not be injected, and you would have separate instances for import(fooSource) and import('/foo.js') in the worker context.

This PR shows that the spec change also leads to a simplification on the host normative constraint as it doesn't then need to specify the injective ordered behaviours.

We have not yet come to a conclusion over which behaviour would be preferable, so I wanted to post this for further discussion.

[nicolo-ribaudo]

These changes look good to me, as discussed in the modules harmony meeting I am ok with this different resolution to the trade off between raciness and cycles handling.