Normative: Define Module Source object identity instead of source text registry identity

[guybedford]

The current specification specifies that import(source1) and import(source2) must return the same instance whenever the URL key matches and ALSO the [[SourceText]] matches between these sources.

This design was justified based on:

1. Defining a concept of instance identity that applies between realms (with an eye to structs proposal)
2. Ensuring that transferring the same module source multiple times to the same agent and importing it would obtain the same instance every time

(2) is not actually such an important aspect of the design - in many cases with separate module source identities (such as sources created via transfer and structured clone), the better design is to support separate instancing and not to unify the instance. In addition it's kind of strange that two module declarations with exactly the same source text will always return the same instance namespace despite being different module declarations.

Given that (1) is not longer a design goal at this point, it is worth reconsidering if we still need this invariant or can instead simplify to this new model.

Previously for eval modules and the module declarations case we discussed ways of associating evaluation indexes for disambiguation between module sources created via eval paths, which would need to be used in coordination with the existing invariants. This complexity is hard to build test coverage on and therefore becomes an implementation risk as well.

By unifying on a simpler source object instancing model here (effectively treating the JS object identity of the source phase as the source key instead of the source text) that does not perform source text equality I think this provides a more intuitive and streamlined design that is less prone to implementation bugs.

In addition we fully define the source keying property that import(source) followed by import(id) where id is the string ID pointing to the same underlying module source as import(source) must return the same module instance. This is a key property, which requires the host invariant refactoring work in #59 to be able to define this based on the concept of a Canonical ModuleRequest Record.

• This is based to the editorial PR in Editorial: separate Module Source Record #56.
• To fully define the host invariant module keying for the source phase, this is also based to the normative invariant change ECMA-262 Normative: formalize module keying host invariant predicates and variables #59.

// cc @nicolo-ribaudo

[nicolo-ribaudo]

I think this reasonable, and probably easier to implement.