Editorial: Rework Module Namespace's [[Get]] method

[shvaikalesh]

This PR merely inlines Module Namespace's [[Get]] into its [[GetOwnProperty]] method because:

• [[Get]] doesn't (need to) terminate prototype chain traversal, unlike TypedArray's [[Get]], since module namespace object's prototype is null and it's non-extensible.
• Some Module Namespace's methods like [[HasProperty]] or [[Delete]] are subtly different from ordinary ones: they prevent from calling into [[GetOwnProperty]] that may throw. We should avoid having methods that are totally equivalent to ordinary ones, like [[Get]], so they won't be accidentally ignored by implementors.
• It relieves runtimes from calling into [[Get]] override, and continue performing OrdinaryGet, which is preferred for simplicity / performance reasons.

[syg]

We discussed this on the editor call, and the conclusion is:

• We like the change to get [[Get]] delegating to [[GetOwnProperty]], since that's the usual state of things.
• We'd like to keep an explicit [[Get]] instead of removing it for clarity. We recommend copy/pasting the relevant parts of ordinary objects' [[Get]], with the irrelevant parts omitted. A note that says this is just ordinary [[Get]] with irrelevant parts omitted would be helpful.
• Keep the steps inlined into [[GetOwnProperty]] as you currently have.