Add Alpha level Experimental Package Manager module

Problem/Motivation

To support Automatic Updates and Project Browser in core Package Manager has been create to run staged Composer commands

This issue is currently only a placeholder to allow preparing the contrib version of Package Manager for core. See #3341974: Finalize \Drupal\automatic_updates\Development\Converter script to update core MR

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet