Parcel API: Allow specifying different sources per target

[LekoArts]

🙋 feature request

The package.json API allows specifying different sources per target: https://parceljs.org/features/targets/#package.json%23targets.*.source

{
  "targets": {
    "frontend": {
      "source": "app/index.html"
    },
    "backend": {
      "source": "api/index.js"
    }
  }
}

At the moment this isn't available with the Parcel API.

🤔 Expected Behavior

With the Parcel API you're able to define the source per target in the same way that entries works.

import {Parcel} from '@parcel/core';

let bundler = new Parcel({
  defaultConfig: '@parcel/config-default',
  mode: 'production',
  targets: {
    modern: {
      source: "**/*.js"
    },
    legacy: {
      source: "**/legacy/*.js"
    }
  }
});

😯 Current Behavior

Not available.

💁 Possible Solution

Basically the codepath in

parcel/packages/core/core/src/requests/EntryRequest.js

Lines 201 to 237 in f8d3fc3

	if (target.source != null) {
	targetsWithSources++;
	let targetSources = Array.isArray(target.source)
	? target.source
	: [target.source];
	let i = 0;
	for (let relativeSource of targetSources) {
	let source = path.join(entry, relativeSource);
	let keyPath = `/targets/${targetName}/source${
	Array.isArray(target.source) ? `/${i}` : ''
	}`;
	await assertFile(
	this.options.inputFS,
	entry,
	relativeSource,
	filePath,
	keyPath,
	this.options,
	);
	entries.push({
	filePath: toProjectPath(this.options.projectRoot, source),
	packagePath: toProjectPath(this.options.projectRoot, entry),
	target: targetName,
	loc: {
	filePath: toProjectPath(
	this.options.projectRoot,
	pkg.filePath,
	),
	...getJSONSourceLocation(
	pkg.map.pointers[keyPath],
	'value',
	),
	},
	});
	i++;
	}

also needs to happen for this.options.targets inside resolveEntry function.

Open question about this: What should happen with the top-level entries? Currently it's required but how would that play into this?

[blipper]

Updating the API to make it clear that this can't be done would save time!