Add libdeno.builtinModules

[ry]

This is needed to support builtin modules like

import { open } from "deno"

This is part of the Native ES modules PR #1460 ... But I think it will be easier to review and land separately.

[piscisaureus]

I'm not sure I quite get the point of this. Are we going to have built-in modules? I thought you were absolutely against namespacing, which suggests that there will be at most 1. Or... what am I missing?

[piscisaureus]

Can you explain a little before you land? I think it might be better to just expose 1 internal module and work the other way, create a module before adding global properties.

[ry]

Can you explain a little before you land?

With the deno_resolve_cb interface, there's no way to support the built-in "deno" module, because the resolve callback requires you to return the source code of the resolved module. In the case of the "deno" module, we don't even have source that needs to be compiled, but rather just need to export some already-compiled symbols (from the snapshot).

I think it might be better to just expose 1 internal module and work the other way, create a module before adding global properties.

The problem is that at the libdeno abstraction layer, we don't know anything about the "deno" module (defined in js/deno.ts). I don't believe you can create modules and add exports to them after the fact... That said, the way I've done this here is rather hacky and I'd be very willing to change it if we can find a better way.