AMD modules unnecessarily uses exports when preservedModules is enabled

[mgol]

Expected Behavior / Situation

When compiling source from ES modules to AMD with --preserveModules, I'd expect default exports in the module to be transformed into unwrapped return values in the resulting AMD module. An example:

Input:

export default window.document;

Output:

define( function() {
	"use strict";

	return window.document;
} );

Actual Behavior / Situation

The module using a default export is converted to one putting what was exported into exports.default:

Input:

export default window.document;

Output:

define(['exports'], function (exports) { 'use strict';

	var document = window.document;

	exports.default = document;

});

Check with Rollup 1.25.2 & 1.27.5.

Modification Proposal

At least when a module doesn't use named exports but only default ones, I'd expect the generated AMD modules to be using the usual return-style exports.

This is, in particular, important to jQuery which wants to transform its new ES modules-based source back to AMD keeping previous idiomatic semantics of those modules.

Ref #2979 (comment)

[lukastaegert]

Fix at #3265