Tooling in the ecosystem have been using the __esModule property to
recognize transpiled ESM in consuming code. For example, a 'log'
package written in ESM:

export function log(val) { console.log(val); }

Can be transpiled as:

exports.__esModule = true;
exports.default = function log(val) { console.log(val); }

The consuming code may be written like this in ESM:

import log from 'log'

Which gets transpiled to:

const _mod = require('log');
const log = _mod.__esModule ? _mod.default : _mod;

So to allow transpiled consuming code to recognize require()'d real ESM
as ESM and pick up the default exports, we add a __esModule property by
building a source text module facade for any module that has a default
export and add .__esModule = true to the exports. We don't do this to
modules that don't have default exports to avoid the unnecessary
overhead. This maintains the enumerability of the re-exported names
and the live binding of the exports.

The source of the facade is defined as a constant per-isolate property
required_module_facade_source_string, which looks like this

export * from 'original';
export { default } from 'original';
export const __esModule = true;

And the 'original' module request is always resolved by
createRequiredModuleFacade() to wrap which is a ModuleWrap wrapping
over the original module.