ComposerFixtureCreator need to move and updated for core merge request

Problem/Motivation

Follow-up to #3341974: Finalize \Drupal\automatic_updates\Development\Converter script to update core MR
I think for the Core Alpha version of Package Manager it is ok to have package_manager/tests/fixtures/fake_site as it is but for beta we should also have the script to recreate it.

Proposed resolution

the core version of ComposerFixtureCreator will need

1. to be moved to core/scripts I guess?

   Right now core's composer.json has

    "autoload-dev": {
           "psr-4": {
               "Drupal\\Composer\\": "composer"
           }
       },

   Not sure if we could just add "Drupal\\PackageManager\\": "core/modules/package_manager/scripts"

   Are modules not suppose to have their own development scripts?
   I see core/scripts/js/ckeditor5-check-plugins.js so this doesn't live in the module. But this is JS script with the composer.json able to set autoload-dev it would seem pretty easy for our script to live in our module

2. Instead of using a script argument to find base of core just use the relative path from where ever the script is moved

Remaining tasks