Realm-agnostic ModuleSource

[guybedford]

If we want to support importability of module sources passed from one Realm to another without structured clone, we will need to have a Realm-agnostic backing record that can be used to achieve this.

Ideally the way this might work out could be an editorial-only refactoring to the spec, that does this separation in a way that module compilation records can be separate from module records. Then to further base the spec on top of this when that is ready.

We would like to try and take this approach where the refactoring is then a Stage 2.7 concern. This issue will be updated with further feedback as we share this plan.