Use placeholder expression when replacing unused symbols

[mischnic]

This is a case where our currently limited form of inner-module tree shaking determined b and b2 to be unused (since only a was imported). But then we still have the var $id$export$id = $id$import$b; declaration in the output.

import * as a from "./a";
import b from "./b";
export { a, b };
export var b2 = b;

The RHS was previously replaced with a reference to an asset that doesn't exist (because is was skipped).

parcelRequire.register("9lh8q", function(module, exports) {
	$parcel$export(module.exports, "a", () => (parcelRequire("e0WFk")));
	var $e0WFk = parcelRequire("e0WFk");

        // unused `export var b2 = b;`
	var $6cd35dda6492a1a0$export$2ec5befb9e6c97d4 = (0, $3nA9q.default);
});

After:

parcelRequire.register("9lh8q", function(module, exports) {
	$parcel$export(module.exports, "a", () => (parcelRequire("e0WFk")));
	var $e0WFk = parcelRequire("e0WFk");

	var $6cd35dda6492a1a0$export$2ec5befb9e6c97d4 = (0, {});
});

(Also note that this is not the common case, for unused export { ... } declarations, there is nothing in the asset and the packager doesn't generate the $parcel$export call in the first place. So there is no problematic placeholder as in the added test case).

[parcel-benchmark]

Benchmark Results

Kitchen Sink ✅

Timings

Description	Time	Difference
Cold	1.90s	-28.00ms
Cached	390.00ms	-18.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

React HackerNews ✅

Timings

Description	Time	Difference
Cold	11.87s	-49.00ms
Cached	523.00ms	-18.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

AtlasKit Editor ✅

Timings

Description	Time	Difference
Cold	2.07m	+206.00ms
Cached	3.13s	+262.00ms ⚠️

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

Three.js ✅

Timings

Description	Time	Difference
Cold	8.54s	-79.00ms
Cached	326.00ms	-26.00ms 🚀

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

Click here to view a detailed benchmark overview.

[devongovett]

Good to merge now @mischnic?