Consider removing GetModuleSource()

[guybedford]

Now that ESM Phase Imports have implemented an explicit [[ModuleSource]] internal field on module records, it may be possible to just define this field upfront for all module records that want to provide their source phase.

Error customization for module sources that don't expose this phase could be as simple as "Module source kind X doesn't implement the source phase", as opposed to needing these being custom completions.

I believe this change can be considered editorial as part of the Stage 4 process for this spec, but this needs to be discussed further.

[guybedford]

Landed in #69.