[V2]Support "exports" at package.json

[YFengFly]

English is not my native language; please excuse typing errors.
I try to use "package","exports","Package Entry Points " as keywords to search issues.Maybe because I'm not good at English, I didn't find the same issues. I'm sorry if this issues is repeated.

🙋 Feature request

Could support "exports" at package.json -- Node.js : Package Entry Points.

In Snabbdom v1 ，it provides some examples:
import { init } from 'snabbdom/init'

But when I use Parcel 1.12.4,it report

Cannot resolve dependency 'snabbdom/init'

In Parcel:2.0.0-beta.1，it report:

Build failed.
@parcel/core: Failed to resolve 'snabbdom/init' from './src/index.js'

For more detail,you can read this issues.

I noticed that in Node.js v12,it provides "exports" key in package.json, and Webpack 5 has already support it(But Webpack 4 doesn't support it yet).Maybe it better to support it.

[mischnic]

Duplicate of #4155