Support dynamic import in bundles.

[kitsonk]

Fixes #4062

[ry]

Why would someone want to have a static module specifier in dynamic import? Wouldn't they just use a normal import?

[kitsonk]

@ry conditional imports. You might include two different modules, but only load one at runtime. Basically this PR looks to see if the dynamic import module specifier is in the bundle, if it is, it uses that, otherwise passes it to import().

I realised there is a defect with this, as I don't instantiate a dynamically imported module property. The test works because the module I am importing doesn't have imports itself.

[kitsonk]

@ry ok, everything is fixed now.

Also, again a practical example:

const myAPIs = "Deno" in globalThis ? await import("./deno_api.ts") : await import("./browser_api.ts");

The compiler would add both deno_api.ts and browser_api.ts to the bundle (and actually set the type of myAPIs for type checking), but at runtime, only the correct module would be instantiated.

[ry]

LGTM