fix: re-exports compiled by tsc not working

[GabbeV]

Motivation

The getOwnPropertyDescriptor method of the exports proxy seems to be incorrect causing export * compiled to commonjs by the typescript compiler to not work.

Typescript will compile export * to the following:

var __exportStar = (this && this.__exportStar) || function(m, exports) {
    for (var p in m) if (p !== "default" && !Object.prototype.hasOwnProperty.call(exports, p)) __createBinding(exports, m, p);
};

Because getOwnPropertyDescriptor always return an object the hasOwnProperty call will always return true causing no exports to get added.

Here is a cut down example of this causing issues when using linaria together with @fluentui/react https://stackblitz.com/edit/linaria-bug-nl1c3w

Summary

The only change made in this PR is to only return a property descriptor when the export exist.

Test plan

I added a test that tests verifies that export * compiled to commonjs by tsc works. The test fails without my change but works with it.