Dynamic import enhanced

[mununki]

A few more points to consider include:

1. Destructuring assignment for dynamic import of value
   It is said that modern bundlers perform tree-shaking for expressions like let {forEach} = ... If possible, it might be worth generating JavaScript output like var {forEach} = ...

2. The current implementation uses the module_system to find the path:string corresponding to the module_id, but it does not consider cases when there are multiple module_systems. Also, within the Js_name_of_module_id.string_of_module_id function, Js_packages_state.get_packages_info () is being used. The same function was used in the place where Pimport was compiled.

[cristianoc]

Not sure I understand the multiple module systems. I thought there's only one in bsconfig and that's propagated to all dependencies?

[mununki]

I see it's modeled as a module_system list here https://github.com/rescript-lang/rescript-compiler/blob/master/jscomp/core/js_packages_info.ml#L57-L67, which I'm not sure about because I'm not familiar with it, so I'll have to look into the compiler logic a bit more.

[mununki]

For 1, the compiler already does a great job of reduction, but I think it's worth comparing the bundling results of the following two pieces of code.

var forEach = await import("..").then(function (m) {return m.forEach;})

var {forEach} = await import("..")