bar.js

import foo from 'foo';

console.log( 'Executed script module bar.' );

export default {};

baz.js

import bar from 'bar';

console.log( 'Executed script module baz.' );

export default {};

foo.js

console.log( 'Executed script module foo.' );

export default {};

try-script-module-priorities.php

<?php
/**
 * Plugin Name: Try Script Module Priorities
 * Author: Weston Ruter
 * Plugin URI: https://github.com/WordPress/gutenberg/pull/70173
 * Update URI: https://gist.github.com/westonruter/b4a1040de3b78fb28d56b9c8cab7da3c
 * Gist Plugin URI: https://gist.github.com/westonruter/b4a1040de3b78fb28d56b9c8cab7da3c
 * Primary Branch: main
 */

namespace TryScriptModulePriorities;

function is_valid_fetchpriority( $priority ) {
	return in_array( $priority, array( 'low', 'high' ), true );
}

function enqueue() {
	$priority_overrides = array();
	foreach ( array( 'foo', 'bar', 'baz' ) as $id ) {
		if ( isset( $_GET['module_fetchpriorities'] ) && is_array( $_GET['module_fetchpriorities'] ) && isset( $_GET['module_fetchpriorities'][ $id ] ) && is_valid_fetchpriority( $_GET['module_fetchpriorities'][ $id ] ) ) {
			$priority_overrides[ $id ] = $_GET['module_fetchpriorities'][ $id ];
		}
	}

	wp_register_script_module(
		'foo',
		plugins_url( 'foo.js', __FILE__ ),
		array(),
		null,
		array(
			'fetchpriority' => $priority_overrides['foo'] ?? 'low',
		)
	);
	wp_register_script_module(
		'bar',
		plugins_url( 'bar.js', __FILE__ ),
		array( 'foo' ),
		null,
		array(
			'fetchpriority' => $priority_overrides['bar'] ?? 'low',
		)
	);
	wp_register_script_module(
		'baz',
		plugins_url( 'baz.js', __FILE__ ),
		array(
			array(
				'id'     => 'bar',
				'import' => 'static',
			),
		),
		null,
		array(
			'fetchpriority' => $priority_overrides['baz'] ?? 'high',
		)
	);

	if ( ! isset( $_GET['modules_enqueued'] ) || ! is_array( $_GET['modules_enqueued'] ) ) {
		wp_enqueue_script_module( 'baz' );
	} else {
		foreach ( array( 'foo', 'bar', 'baz' ) as $id ) {
			if ( in_array( $id, $_GET['modules_enqueued'], true ) ) {
				wp_enqueue_script_module( $id );
			}
		}
	}
}

add_action( 'wp_enqueue_scripts', __NAMESPACE__ . '\\enqueue' );