Editorial: Add a note about the possibility of deferring getting the list of exports

[nicolo-ribaudo]

A goal of the proposal is that implementations can, under some circumstances, easily implement lazy loading of deferred modules (when they know that they don't have syntax errors or transitive async dependencies). While trying to actually implement an example of that, I found a problem: even though it's not user-exposed, the spec eagerly reads the list of exports in GetModuleNamespace.

This PR centralized the logic to get and sort the list of names in a single place, and adds a note explaining that it's a pure operation and thus can actually be deferred if needed for implementation purposes.

Closes #68

[guybedford]

This does seem preferable to #68, noting laziness as a property implementations can use as opposed to trying to hard code it.